# 第6章：直接操纵merge

## 基础知识

从物理结构上讲，一个commit表示一个完整的版本； 但是，从逻辑结构上讲，一个commit还可以表示 *相比于之前进行了哪些修改* 。

本章中涉及worktree的命令会明确标出。

```bash
git init --separate-git-dir "$(pwd)" ../default-tree
# Initialized empty Git repository in /root/
```

## 查看更改

在开始之前，先创建几个对象：

```bash
echo '1' | git hash-object -t blob --stdin -w
# d00491fd7e5bb6fa28c517a0bb32b8b506539d4d
echo '2' | git hash-object -t blob --stdin -w
# 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f
echo '3' | git hash-object -t blob --stdin -w
# 00750edc07d6415dcc07ae0351e9397b0222b7ba
echo '4' | git hash-object -t blob --stdin -w
# b8626c4cff2849624fb67f87cd0ad72b163671ad
echo '5' | git hash-object -t blob --stdin -w
# 7ed6ff82de6bcc2a78243fc9c54d3ef5ac14da69
echo '6' | git hash-object -t blob --stdin -w
# 1e8b314962144c26d5e0e50fd29d2ca327864913
echo '7' | git hash-object -t blob --stdin -w
# 7f8f011eb73d6043d2e6db9d2c101195ae2801f2
git mktree <<EOF
100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d$(printf '\t')1.txt
100755 blob 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f$(printf '\t')2.txt
EOF
# a237e8338c09e7d1b2f9749f73f4f583f19fc626
git mktree <<EOF
100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d$(printf '\t')1.txt
100755 blob 00750edc07d6415dcc07ae0351e9397b0222b7ba$(printf '\t')3.txt
EOF
# aa250e2798646facc12686e4403ccadbf1565d51
git hash-object -t commit --stdin -w <<EOF
tree a237e8338c09e7d1b2f9749f73f4f583f19fc626
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

1=1 2=2
EOF
# 4cfe841426d0435270d049625a766130c108f4c8
git hash-object -t commit --stdin -w <<EOF
tree aa250e2798646facc12686e4403ccadbf1565d51
parent 4cfe841426d0435270d049625a766130c108f4c8
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

1=1 3=3
EOF
# afc38c96c82ea65991322a5d28995b0851ff7edd
```

* Lv2

基于第一个tree-ish，查看第二个tree-ish的修改：

```bash
git diff-tree a237 aa25
# :100755 000000 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 0000000000000000000000000000000000000000 D    2.txt
# :000000 100755 0000000000000000000000000000000000000000 00750edc07d6415dcc07ae0351e9397b0222b7ba A    3.txt
git diff-tree a237 aa25 -- 2.txt
# :100755 000000 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 0000000000000000000000000000000000000000 D    2.txt
git diff-tree afc3
# afc38c96c82ea65991322a5d28995b0851ff7edd
# :100755 000000 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 0000000000000000000000000000000000000000 D    2.txt
# :000000 100755 0000000000000000000000000000000000000000 00750edc07d6415dcc07ae0351e9397b0222b7ba A    3.txt
```

基于tree-ish，查看index的修改：

```bash
git read-tree a237
git diff-index --cached aa25
# :000000 100755 0000000000000000000000000000000000000000 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f A    2.txt
# :100755 000000 00750edc07d6415dcc07ae0351e9397b0222b7ba 0000000000000000000000000000000000000000 D    3.txt
```

基于tree-ish，查看worktree的修改：

```bash
rm -rf ../default-tree/*
git --work-tree=../default-tree diff-index aa25
# :100644 000000 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0000000000000000000000000000000000000000 D    1.txt
# :100755 000000 00750edc07d6415dcc07ae0351e9397b0222b7ba 0000000000000000000000000000000000000000 D    3.txt
```

基于index，查看worktree的修改：

```bash
git --work-tree=../default-tree diff-files
# :100644 000000 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0000000000000000000000000000000000000000 D    1.txt
# :100755 000000 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 0000000000000000000000000000000000000000 D    2.txt
```

* Lv3
* `git diff <tree-ish> <tree-ish> -- [<path>]` 相当于 `git diff-tree -p <tree-ish> <tree-ish> -- [path]`
* `git show <commit-ish> -- [<path>]` 相当于 `git diff-tree -p <commit-ish> -- [<path>]`
* `git diff [<tree-ish>] -- [<path>]` 相当于 `git diff-index -p [<tree-ish>] -- [<path>]`
* `git diff --cached [<tree-ish>] -- [<path>]` 相当于 `git diff-index -p --cached [<tree-ish>] -- [<path>]`
* `git diff -- <path>` 相当于 `git diff-files -p <path>`
* `git status` 相当于 `git diff-index HEAD`、`git diff-index --cached HEAD`、`git clean -nd`
  * 注意这里没有`-p`，意味着仅仅列出哪些文件发生了变化，但并不列出具体差异
  * `git clean -nd`意味着还要列出新增了哪些文件
* Lv4: `git st`

`git status -sb`，比起`git status`要简明扼要一些。

## 处理修改

类似于`git bundle create`将若干对象打包成字节流以便离线传送，`git diff-* -p|--patch`将修改打包成字节流以便离线传送。 类似于`git bundle unbundle`将字节流解包成对象，`git apply`将字节流解包出修改。 需要注意的是，`--patch`产生的是human-readable data，但`git bundle`是machine-readable only。

打包修改：

```bash
git diff-tree --patch a237 aa25 | tee ../the.patch
# diff --git a/2.txt b/2.txt
# deleted file mode 100755
# index 0cfbf08..0000000
# --- a/2.txt
# +++ /dev/null
# @@ -1 +0,0 @@
# -2
# diff --git a/3.txt b/3.txt
# new file mode 100755
# index 0000000..00750ed
# --- /dev/null
# +++ b/3.txt
# @@ -0,0 +1 @@
# +3
```

解包修改至worktree:

```bash
rm -rf ../default-tree/*
git read-tree a237
git --work-tree=../default-tree checkout-index -fu -a
ls ../default-tree
# 1.txt
# 2.txt
git -C ../default-tree apply ../the.patch
ls ../default-tree
# 1.txt
# 3.txt
git ls-files -s
# 100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0    1.txt
# 100755 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 0    2.txt
```

解包修改至index：

```bash
git ls-files -s
# 100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0    1.txt
# 100755 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 0    2.txt
git apply --cached ../the.patch
git ls-files -s
# 100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0    1.txt
# 100755 00750edc07d6415dcc07ae0351e9397b0222b7ba 0    3.txt
ls ../default-tree
# 1.txt
# 3.txt
```

## Merge相关概念简介

Merge是git里面最为复杂也最为重要的部分。 在具体讲解每一个命令之前，先看一下Lv2的big picture：

* 同一份文件，已知原始版本，如何把两人独自进行的修改整合？
  * 出现冲突时以我方为准：`git merge-file --ours C A B`
  * 出现冲突时以对方为准：`git merge-file --theirs C A B`
  * 出现冲突时串联起来：`git merge-file --union C A B`
  * 出现冲突时做出标记，手工解决：`git merge-file C A B`
  * 采用其他工具：（略）
* 同一个tree，已知原始版本，如何把两人独自进行的修改在文件层面上整合？（不涉及文件内容）
  * `git merge-tree C A B`
* 同一个tree，已知原始版本，对方修改以后，如何把我尚未完善的修改建立在别人的基础上？
  * `git read-tree -m H M`
* 同一个tree，已知原始版本，如何把两人独自进行的修改整合？
  * 先解决最简单的冲突，保留复杂冲突的完整信息：`git read-tree -m A C B`
  * 试着解决最简单的几种冲突：`git merge-index git-merge-one-file`
* 同一个tree，已知原始版本，如何把多人独自进行的修改整合？
  * 多次执行`git read-tree -m A C B`即可

## 3-Way merge基本概念之文件层面的`git merge-file`

重要Lv2工具`git merge-file`（无需git dir也无需worktree）：

```bash
cat >fileA <<EOF
lineB
...some stuff...
lineC
EOF
cat >fileB <<EOF
lineBB
...some stuff...
lineC
EOF
cat >fileC <<EOF
lineB
...some stuff...
lineCC
EOF
cat >fileD <<EOF
lineBD
...some stuff...
lineC
EOF
# 计算 fileC + (fileB - fileA)
git merge-file --stdout fileC fileA fileB
# lineBB
# ...some stuff...
# lineCC
```

在遇到冲突的情况下，标记出来手工解决：

```bash
git merge-file --stdout fileD fileA fileB
# <<<<<<< fileD
# lineBD
# =======
# lineBB
# >>>>>>> fileB
# ...some stuff...
# lineC
git merge-file --stdout --diff3 fileD fileA fileB
# <<<<<<< fileD
# lineBD
# ||||||| fileA
# lineB
# =======
# lineBB
# >>>>>>> fileB
# ...some stuff...
# lineC
```

在遇到冲突的情况下，自动解决：

```bash
git merge-file --stdout --our fileD fileA fileB
# lineBD
# ...some stuff...
# lineC
git merge-file --stdout --their fileD fileA fileB
# lineBB
# ...some stuff...
# lineC
git merge-file --stdout --union fileD fileA fileB
# lineBD
# lineBB
# ...some stuff...
# lineC
```

### tree层面：`git merge-tree`

在开始之前，先创建几个对象：

```bash
git mktree <<EOF
100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d$(printf '\t')1.txt
100755 blob 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f$(printf '\t')2.txt
100755 blob b8626c4cff2849624fb67f87cd0ad72b163671ad$(printf '\t')4.txt
EOF
# 5de99716b8dd347ce09718e5f628b8c78e656b8c
git hash-object -t commit --stdin -w <<EOF
tree 5de99716b8dd347ce09718e5f628b8c78e656b8c
parent 4cfe841426d0435270d049625a766130c108f4c8
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

1=1 2=2 4=4
EOF
# d2b78d09d49d1f9b4ae260cf98657de9a2fedaa6
```

`git merge-tree C A B`的意义是`(C+(B-A))-C`，参见以下示例

```bash
git ls-tree a237
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f    2.txt
git ls-tree aa25
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob 00750edc07d6415dcc07ae0351e9397b0222b7ba    3.txt
git ls-tree 5de9
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f    2.txt
# 100755 blob b8626c4cff2849624fb67f87cd0ad72b163671ad    4.txt
git merge-tree 5de9 a237 aa25
# removed in remote
#   base   100755 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 2.txt
#   our    100755 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 2.txt
# @@ -1 +0,0 @@
# -2
# added in remote
#   their  100755 00750edc07d6415dcc07ae0351e9397b0222b7ba 3.txt
# @@ -0,0 +1 @@
# +3
git merge-tree 5de9 aa25 a237
git merge-tree a237 aa25 5de9
# added in remote
#   their  100755 b8626c4cff2849624fb67f87cd0ad72b163671ad 4.txt
# @@ -0,0 +1 @@
# +4
```

当同时存在tree层面和文件层面的修改时，参见以下示例

```bash
# 注意：3.txt包含的是"4"
git mktree <<EOF
100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d$(printf '\t')1.txt
100755 blob b8626c4cff2849624fb67f87cd0ad72b163671ad$(printf '\t')3.txt
EOF
# 47e3b7857c03c35eae515b36fe3828ef073cc2aa
# merge-tree产生非常规输出（removed in local），暗示需要处理冲突
git merge-tree 47e3 a237 aa25
# removed in local
#   base   100755 b8626c4cff2849624fb67f87cd0ad72b163671ad 3.txt
#   their  100755 00750edc07d6415dcc07ae0351e9397b0222b7ba 3.txt
```

### `git read-tree -m`的Two Tree Merge

`git read-tree -m <tree-ish-H> <tree-ish-M>`试图执行`index=index+(M-H)`。 对于每一个文件，考虑到H、M、index、worktree四处的状态，有22种具体情况，具体处置方法如下表所示（0表示不存在，不同小写字母表示不同版本）（摘自`man git-read-tree`）：

| 编号  | worktree | index | H | M | 处置         |      |
| --- | -------- | ----- | - | - | ---------- | ---- |
| 0   | .        | 0     | 0 | 0 | 0          | keep |
| 1   | .        | 0     | 0 | m | index = m  |      |
| 2   | .        | 0     | h | 0 | index = 0  |      |
| 3a  | .        | 0     | h | h | *SEE NOTE* |      |
| 3b  | .        | 0     | h | m | fail       |      |
| 4   | i        | i     | 0 | 0 | keep       |      |
| 5   | w        | i     | 0 | 0 | keep       |      |
| 6   | m        | m     | 0 | m | keep       |      |
| 7   | w        | m     | 0 | m | keep       |      |
| 8   | i        | i     | 0 | m | fail       |      |
| 9   | w        | i     | 0 | m | fail       |      |
| 10  | h        | h     | h | 0 | index = 0  |      |
| 11  | w        | h     | h | 0 | fail       |      |
| 12  | i        | i     | h | 0 | fail       |      |
| 13a | h        | i     | h | 0 | fail       |      |
| 13b | w        | i     | h | 0 | fail       |      |
| 14  | h        | h     | h | h | keep       |      |
| 15  | w        | h     | h | h | keep       |      |
| 16  | i        | i     | h | m | fail       |      |
| 17a | h        | i     | h | m | fail       |      |
| 17b | m        | i     | h | m | fail       |      |
| 17c | w        | i     | h | m | fail       |      |
| 18  | m        | m     | h | m | keep       |      |
| 19a | h        | m     | h | m | keep       |      |
| 19b | w        | m     | h | m | keep       |      |
| 20  | h        | h     | h | m | index = m  |      |
| 21a | m        | h     | h | m | fail       |      |
| 21b | w        | h     | h | m | fail       |      |

对于3a，如果index全空，则执行index=m；若index非空，则keep。

举例如下：

```bash
rm -rf ../default-tree/*
git read-tree 5de9
# 此处要加-u，来确保index和worktree在stat意义下一致
git --work-tree=../default-tree checkout-index -fu -a
git ls-files -s
# 100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0    1.txt
# 100755 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 0    2.txt
# 100755 b8626c4cff2849624fb67f87cd0ad72b163671ad 0    4.txt
ls ../default-tree
# 1.txt
# 2.txt
# 4.txt
git ls-tree a237
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f    2.txt
git ls-tree aa25
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob 00750edc07d6415dcc07ae0351e9397b0222b7ba    3.txt
git --work-tree=../default-tree read-tree -m a237 aa25
git ls-files -s
# 100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0    1.txt
# 100755 00750edc07d6415dcc07ae0351e9397b0222b7ba 0    3.txt
# 100755 b8626c4cff2849624fb67f87cd0ad72b163671ad 0    4.txt
ls ../default-tree
# 1.txt
# 2.txt
# 4.txt
# 总结：
# 1.txt hhhh -> keep (#14)
# 2.txt hhh0 -> index = 0 (#10)
# 3.txt 000m -> index = m (#1)
# 4.txt ii00 -> keep (#4)
```

### `git read-tree -m`的3-Way Merge

`git read-tree -m [--aggressive] <tree-ish-A> <tree-ish-C> <tree-ish-B>`的意思是：

* 清空index
* stage数=1，读取`<tree-ish-A>`至index
* stage数=2，读取`<tree-ish-C>`至index
* stage数=3，读取`<tree-ish-B>`至index
* 按下表变换（表中use表示删掉stage123的，创建一个新的stage0条目），若不能变换则不处理

| `--aggressive` | stage1 | stage2 | stage3 | 操作    |
| -------------- | ------ | ------ | ------ | ----- |
| any            | a      | a      | a      | use a |
| any            | a      | x      | x      | use x |
| any            | a      | a      | b      | use b |
| any            | a      | c      | a      | use c |
| required       | a      | 0      | a      | use 0 |
| required       | a      | a      | 0      | use 0 |
| required       | a      | 0      | 0      | use 0 |
| required       | 0      | x      | x      | use x |

举例如下：

```bash
git ls-tree a237
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f    2.txt
git ls-tree 47e3
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob b8626c4cff2849624fb67f87cd0ad72b163671ad    3.txt
git ls-tree aa25
# 100644 blob d00491fd7e5bb6fa28c517a0bb32b8b506539d4d    1.txt
# 100755 blob 00750edc07d6415dcc07ae0351e9397b0222b7ba    3.txt
rm -rf ../default-tree/*
git read-tree 47e3
git --work-tree=../default-tree checkout-index -fu -a
git --work-tree=../default-tree read-tree -m a237 47e3 aa25
# 此时可以看见非0的stage数
git ls-files -s
# 100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0    1.txt
# 100755 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f 1    2.txt
# 100755 b8626c4cff2849624fb67f87cd0ad72b163671ad 2    3.txt
# 100755 00750edc07d6415dcc07ae0351e9397b0222b7ba 3    3.txt
# 总结：
# 1.txt aaa, collapse
# 2.txt a00, non-collapse
# 3.txt 0cb, non-collapse
# 此时无法write-tree：
git write-tree
# 2.txt: unmerged (0cfbf08886fca9a91cb753ec8734c84fcbe52c9f)
# 3.txt: unmerged (b8626c4cff2849624fb67f87cd0ad72b163671ad)
# 3.txt: unmerged (00750edc07d6415dcc07ae0351e9397b0222b7ba)
# fatal: git-write-tree: error building trees
git --work-tree=../default-tree checkout-index --stage=all -f --all
# .merge_file_bL417W . .    2.txt
# . .merge_file_m91Yo2 .merge_file_FwwWF7    3.txt
```

### `git merge-index`和`git merge-one-file`

前述`git read-tree`只解决了最最简单的冲突。为了解决更多冲突，要么手工编辑好再`git update-index`或者`git add`，要么采用自动化工具。

`git merge-index`负责调用其他工具：

```bash
# 注意空格
git merge-index echo -a
# 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f   2.txt 100755  
#  b8626c4cff2849624fb67f87cd0ad72b163671ad 00750edc07d6415dcc07ae0351e9397b0222b7ba 3.txt  100755 100755
```

一种（没有卵用的）操作是调用自带工具`git-merge-one-file`：

```bash
rm -rf ../default-tree/*
git read-tree 47e3
git --work-tree=../default-tree checkout-index -fu -a
git --work-tree=../default-tree read-tree -m --aggressive a237 47e3 aa25
git ls-files -s
# 100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0    1.txt
# 100755 b8626c4cff2849624fb67f87cd0ad72b163671ad 2    3.txt
# 100755 00750edc07d6415dcc07ae0351e9397b0222b7ba 3    3.txt
# 这个自带工具非常弱，解决不了什么问题，真心不比--aggressive强多少
git --work-tree=../default-tree merge-index git-merge-one-file -a
# fatal: unable to read blob object e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
# error: Could not stat : No such file or directory
# ERROR: content conflict in 3.txt
# fatal: merge program failed
# Added 3.txt in both, but differently.
```

注意：若提示错误 `fatal: unable to read blob object e69de29bb2d1d6434b8b29ae775ad8c2e48c5391` 则执行 `printf '' | git hash-object --stdin -w`

* 问题：在使用外部工具的情况下，有没有更好的解决冲突的办法？
* 回答：有，即著名的recursive merge。但是由于该算法非常复杂，没有Lv2命令。此处不介绍Lv1实现，而只介绍Lv3的使用方法。

## 原始版本的选择：`git merge-base`

为了减少花在查找原始版本（A）的努力，`git merge-base -a <commit>*`可以直接计算出多个commit的极近公共祖先（在偏序关系下没有“最”，只能有“极”）。

```bash
git merge-base -a afc3 d2b7
# 4cfe841426d0435270d049625a766130c108f4c8
```

## Lv3

`git merge -s <strategy>`（除了recursive、subtree、ours以外）实际上就是调用`git merge-<strategy>`。

而所谓的subtree merge，本质上就是subtree shift非空的recursive merge。 git会自动计算给B添加的prefix，但也可以通过`-Xsubtree=`来手动指定。

`git merge -s resolve --no-ff --no-commit`实际上就是 （参见[`git-merge-resolve`源码](https://github.com/git/git/blob/master/git-merge-resolve.sh)）

* （需要手工指定C）
* 先执行`git read-tree -u -m --aggressive A C B`
* 再执行`git merge-index -o git-merge-one-file -a`
* 在git dir中生成`MERGE_HEAD` `MERGE_MODE` `MERGE_MSG`三个文件

`git merge -s octopus --no-ff --no-commit`实际上就是 （参见[`git-merge-octopus`源码](https://github.com/git/git/blob/master/git-merge-octopus.sh)）

* 用`git merge-base`算出C
* 先执行`git read-tree -u -m --aggressive A C B`
* 再执行`git merge-index -o git-merge-one-file -a`
* 对其他分支重复以上过程
* 在git dir中生成`MERGE_HEAD` `MERGE_MODE` `MERGE_MSG`三个文件

`git merge --no-ff --no-commit - ours`实际上就是

* 关于index，什么也不做
* 在git dir中生成`MERGE_HEAD` `MERGE_MODE` `MERGE_MSG`三个文件

`git merge --no-ff --no-commit -s recursive`的基本思路是 （参见`man git-merge`）

* 先用`git merge-base --all`找到所有可能的C
* 把这些C先行merge一下得到C'
* 考虑重命名，对A C' B进行3-way merge

  关于recursive，还有很多的参数可以调节，比如
* `-Xours`类似于`git merge-file --ours`
* `-Xtheirs`类似于`git merge-file -theirs`
* `-Xsubtree`指定给B加什么前缀再进行recursive merge

若有`--ff`则表明：若A=C或A=B，则不做任何事情。

若有`--no-commit`则表示：若无冲突，则自动执行`git commit`

需要注意的是，在`MERGE_HEAD`等文件存在时，`git commit`调用`git commit-tree`时会自动带上几个`-p`参数，生成多个parent的commit，其中第1个parent是HEAD，余下的是`MERGE_HEAD`的内容，也即`git merge`的参数。

## Lv4

由于很多时候我们希望主动控制要不要创建merge commit（也即手动决定`--ff-only`或者`--no-ff`，且希望在commit之前仔细检查合并是否正确（如跑单元测试），故本文建议使用如下alias：

```
alias.mf=merge --ff-only
alias.mnf=merge --no-ff
alias.mnfnc=merge --no-ff --no-commit
```

其中`git mf`用于`git fetch`之后，`git mnf`用于日常merge其他简单分支，而`git mnfnc`用于尝试merge（也即`git read-tree -u -m`）、跑单元测试再commit的情况。

## Lv5

有一类merge情况是，需要用其他分支 *完全取代* 当前分支的某一目录。（第12章整章建立在此基础之上） 然而，即便`git merge --no-ff -s subtree -Xsubtree=<prefix>`有时也会出错（毕竟是`git read-tree -m`）。

采用以下脚本即可解决：

```bash
git-mnfss() {
  git rm --cached -r -- $1
  git read-tree --prefix $1/ $1
  git checkout-index -fua
  git clean -f -- $1
  git reset --soft $(echo "Merge branch $1" | git commit-tree $(git write-tree) -p HEAD -p $1)
}
```

## 关于merge信息的完整性

在执行`git merge --no-ff B*`的时候，新创建的commit中包括多个parent，用来记录谁和谁进行了merge。 这是为了能够在以后追溯到当初merge时候的细节。 然而受限于parent的格式，parent只能记录哪些commit进行了merge，与commit相关的其他信息就无法记录了： 附于该commit上的note，指向该commit的tag(s)，以及该commit所属的refs。

* 对于notes，可以直接忽略，因为note里面的信息几乎都是本机使用
* 对于tag，存储于新创建的commit的message body中
* 对于带有签名（见第13章）的tag，存储于新创建的commit的mergetag中
* 对于ref，存储于message中（Merge xxx branch）

首先创建若干空commit和tag：

```bash
printf '' | git mktree
# 4b825dc642cb6eb9a060e54bf8d69288fbee4904
git hash-object -t commit --stdin -w <<EOF
tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

A
EOF
# 6784b23b1a03700628d8adb65b57b5b4816caa01
echo $'100644 blob 0cfbf08886fca9a91cb753ec8734c84fcbe52c9f\tB.txt' | git mktree
# 6f707a369aa805c91f6d737ccda037d137d0ef93
git hash-object -t commit --stdin -w <<EOF
tree 6f707a369aa805c91f6d737ccda037d137d0ef93
parent 6784b23b1a03700628d8adb65b57b5b4816caa01
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

B
EOF
# f1d113e4db427a1824524d17928a2cb53cd5090a
echo $'100644 blob 00750edc07d6415dcc07ae0351e9397b0222b7ba\tC.txt' | git mktree
# f6ab164f6371575aff1ce5c21441835e5568b182
git hash-object -t commit --stdin -w <<EOF
tree f6ab164f6371575aff1ce5c21441835e5568b182
parent 6784b23b1a03700628d8adb65b57b5b4816caa01
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

C
EOF
# 28c0a4a3bab80a464dd384cf4e3d2b83cceb602b
git branch br-C 28c0
echo $'100644 blob b8626c4cff2849624fb67f87cd0ad72b163671ad\tD.txt' | git mktree
# 2cff17f83cfc5f0cd3af03f99257e35906e6fb7c
git hash-object -t commit --stdin -w <<EOF
tree 2cff17f83cfc5f0cd3af03f99257e35906e6fb7c
parent 6784b23b1a03700628d8adb65b57b5b4816caa01
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

D
EOF
# 7f24235935c56e397d2d1d55bb470fe1b01b8209
git tag tag-D 7f24
echo $'100644 blob 7ed6ff82de6bcc2a78243fc9c54d3ef5ac14da69\tE.txt' | git mktree
# 4ff6131a9638bb0a38917c72255c3c0635d0f464
git hash-object -t commit --stdin -w <<EOF
tree 4ff6131a9638bb0a38917c72255c3c0635d0f464
parent 6784b23b1a03700628d8adb65b57b5b4816caa01
author b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800
committer b1f6c1c4 <b1f6c1c4@gmail.com> 1514736000 +0800

E
EOF
# 1a1640224e55b3a7d05108c6b91e03e6cc65ffbe
GIT_COMMITTER_NAME=b1f6c1c4 \
GIT_COMMITTER_EMAIL=b1f6c1c4@gmail.com \
GIT_COMMITTER_DATE='1600000000 +0800' \
git tag -a -m 'Tag for E' tag-obj-E 1a16
```

然后进行merge：

```bash
git update-ref --no-deref HEAD 6784
git -C ../default-tree reset --hard
# HEAD is now at 6784b23 A
git -C ../default-tree clean -fdx
# Removing .merge_file_FwwWF7
# Removing .merge_file_bL417W
# Removing .merge_file_m91Yo2
GIT_AUTHOR_NAME=b1f6c1c4 \
GIT_AUTHOR_EMAIL=b1f6c1c4@gmail.com \
GIT_AUTHOR_DATE='1600000000 +0800' \
GIT_COMMITTER_NAME=b1f6c1c4 \
GIT_COMMITTER_EMAIL=b1f6c1c4@gmail.com \
GIT_COMMITTER_DATE='1600000000 +0800' \
git -C ../default-tree merge --no-ff tag-obj-E f1d1 br-C tag-D
# Fast-forwarding to: tag-obj-E
# Trying simple merge with f1d1
# Trying simple merge with br-C
# Trying simple merge with tag-D
# Merge made by the 'octopus' strategy.
#  B.txt | 1 +
#  C.txt | 1 +
#  D.txt | 1 +
#  E.txt | 1 +
#  4 files changed, 4 insertions(+)
#  create mode 100644 B.txt
#  create mode 100644 C.txt
#  create mode 100644 D.txt
#  create mode 100644 E.txt
git cat-file commit HEAD
# tree ae618f9e9f1a0ce0fdc25f7e4dcfdc5bc9c09c49
# parent 6784b23b1a03700628d8adb65b57b5b4816caa01
# parent 1a1640224e55b3a7d05108c6b91e03e6cc65ffbe
# parent f1d113e4db427a1824524d17928a2cb53cd5090a
# parent 28c0a4a3bab80a464dd384cf4e3d2b83cceb602b
# parent 7f24235935c56e397d2d1d55bb470fe1b01b8209
# author b1f6c1c4 <b1f6c1c4@gmail.com> 1600000000 +0800
# committer b1f6c1c4 <b1f6c1c4@gmail.com> 1600000000 +0800
#
# Merge branch 'br-C', tags 'tag-obj-E' and 'tag-D'; commit 'f1d1' into HEAD
#
# Tag for E
```

注意观察commit message中对于各parent的不同的描述。 另外，commit message中还包含了tag message。

## 总结

* 查看和处理修改
  * Lv2
    * `git diff-tree [-p] <tree-ish> <tree-ish> -- <path>`
    * `git diff-tree [-p] <commit-ish> -- <path>`
    * `git diff-index [-p] [--cached] <tree-ish> -- <path>`
    * `git diff-files [-p] <path>`
    * `git apply [--cached] <patch> -- <path>`
  * Lv3
    * `git diff <tree-ish> <tree-ish> -- <path>`
    * `git show <commit-ish> -- <path>`
    * `git diff [--cached] <tree-ish> -- <path>`
    * `git diff -- <path>`
    * `git status`
  * Lv4
    * `git st`
* 合并修改
  * Lv2
    * `git merge-file [--ours|--theirs|--union] C A B`
    * `git merge-tree C A B`
    * `git read-tree -m H M`
    * `git read-tree -m A C B`
    * `git merge-index -o git-merge-one-file -a`
  * Lv3
    * `git merge -s resolve [--no-ff] [--no-commit] B`
    * `git merge -s octopus [--no-ff] [--no-commit] B*`
    * `git merge -s ours [--no-ff] [--no-commit] B*`
    * `git merge -s recursive [--no-ff] [--no-commit] B`
    * `git merge -s subtree [--no-ff] [--no-commit] B`
  * Lv4
    * `git mf`
    * `git mnf`
    * `git mnfnc`
  * Lv5
    * `git-mnfss`