您可以检查分支A的reflog，以找到它是从哪个提交创建的，以及该分支所指向的提交的完整历史。Reflogs在.git/logs中。

你可以使用下面的命令返回branch_a中最老的提交，master无法访问它:

git rev-list branch_a ^master | tail -1

也许有一个额外的健全性检查，该提交的父节点实际上可以从master访问…

Git 2.36提出了一个更简单的命令:

(branch_A_tag)
     |
--X--A--B--C--D--F  (master) 
      \   / \   /
       \ /   \ /
        G--H--I--J  (branch A)

vonc@vclp MINGW64 ~/git/tests/branchOrigin (branch_A)
git log -1 --decorate --oneline \
  $(git rev-parse \
     $(git rev-list --exclude-first-parent-only ^main branch_A| tail -1)^ \
   )
 80e8436 (tag: branch_A_tag) A - Work in branch main

^main branch_A给出的是J—I—H—G -1得到G git rev-parse G^给你它的第一个父:A或branch_A_tag

使用测试脚本:

mkdir branchOrigin
cd branchOrigin
git init
git commit --allow-empty -m "X - Work in branch main"
git commit --allow-empty -m "A - Work in branch main"
git tag branch_A_tag     -m "Tag branch point of branch_A"
git commit --allow-empty -m "B - Work in branch main"
git switch -c branch_A branch_A_tag
git commit --allow-empty -m "G - Work in branch_A"
git switch main
git merge branch_A       -m "C - Merge branch_A into branch main"
git switch branch_A
git commit --allow-empty -m "H - Work in branch_A"
git merge main         -m "I - Merge main into branch_A"
git switch main
git commit --allow-empty -m "D - Work in branch main"
git merge branch_A       -m "F - Merge branch_A into branch main"
git switch branch_A
git commit --allow-empty -m "J - Work in branch_A branch"

这就给了你:

vonc@vclp MINGW64 ~/git/tests/branchOrigin (branch_A)
$ git log --oneline --decorate --graph --branches --all
* a55a87e (HEAD -> branch_A) J - Work in branch_A branch
| *   3769cc8 (main) F - Merge branch_A into branch main
| |\
| |/
|/|
* |   1b29fa5 I - Merge main into branch_A
|\ \
* | | e7accbd H - Work in branch_A
| | * 87a62f4 D - Work in branch main
| |/
| *   7bc79c5 C - Merge branch_A into branch main
| |\
| |/
|/|
* | 0f28c9f G - Work in branch_A
| * e897627 B - Work in branch main
|/
* 80e8436 (tag: branch_A_tag) A - Work in branch main
* 5cad19b X - Work in branch main

这是:

(branch_A_tag)
     |
--X--A--B--C--D--F  (master) 
      \   / \   /
       \ /   \ /
        G--H--I--J  (branch A)

在Git 2.36 (Q2 2022)中，“Git log”(man)和朋友们学习了一个选项——exclude-first-parent-only只沿着第一个父链向下传播UNINTERESTING位，就像——first-parent选项只显示在第一个父链上缺乏UNINTERESTING位的提交一样。

参见Jerry Zhang (Jerry -skydio)提交的9d505b7 (11 Jan 2022)。 (由Junio C Hamano—gitster—在commit 708cbef中合并，2022年2月17日)

Git-rev-list: add——exclude-first-parent-only标志 署名:Jerry Zhang

It is useful to know when a branch first diverged in history from some integration branch in order to be able to enumerate the user's local changes. However, these local changes can include arbitrary merges, so it is necessary to ignore this merge structure when finding the divergence point. In order to do this, teach the "rev-list" family to accept "--exclude-first-parent-only", which restricts the traversal of excluded commits to only follow first parent links. -A-----E-F-G--main \ / / B-C-D--topic In this example, the goal is to return the set {B, C, D} which represents a topic branch that has been merged into main branch. git rev-list topic ^main(man) will end up returning no commits since excluding main will end up traversing the commits on topic as well. git rev-list --exclude-first-parent-only topic ^main(man) however will return {B, C, D} as desired. Add docs for the new flag, and clarify the doc for --first-parent to indicate that it applies to traversing the set of included commits only.

Rev-list-options现在包括在它的手册页:

——“首席家长 当找到要包含的提交时，只遵循第一个 父节点在看到合并提交时提交。 这个选项 能给一个更好的概述时，查看的演变 一个特定的主题分支，因为合并到一个主题中 分支倾向于只调整到更新的上游 时不时地，这个选项可以让你忽略 个人的承诺被带入你的历史 一个合并。

Rev-list-options现在包括在它的手册页:

——exclude-first-parent-only 当发现提交排除(使用'{插入符号}')时，只跟随 第一个父节点在看到合并提交时提交。 这可用于查找主题分支中的更改集 从它与远分支的分歧点开始 任意的合并可以是有效的主题分支更改。

我使用git rev-list来做这类事情。例如，(注意3个点)

$ git rev-list --boundary branch-a...master | grep "^-" | cut -c2-

将分叉点吐出来。这并不完美;因为你已经多次将master合并到分支A中，这将分离出两个可能的分支点(基本上，最初的分支点，然后是你将master合并到分支A的每个点)。然而，它至少应该缩小可能性。

我将该命令添加到~/中的别名中。gitconfig:

[alias]
    diverges = !sh -c 'git rev-list --boundary $1...$2 | grep "^-" | cut -c2-'

所以我可以称它为:

$ git diverges branch-a master

下面的命令将显示Commit A的SHA1

git merge-base -fork-point A

一个简单的方法是使用选项——first-parent来更容易地看到git log -graph中的分支点。

例如，从已接受的答案中取repo:

$ git log --all --oneline --decorate --graph

*   a9546a2 (HEAD -> master, origin/master, origin/HEAD) merge from topic back to master
|\  
| *   648ca35 (origin/topic) merging master onto topic
| |\  
| * | 132ee2a first commit on topic branch
* | | e7c863d commit on master after master was merged to topic
| |/  
|/|   
* | 37ad159 post-branch commit on master
|/  
* 6aafd7f second commit on master before branching
* 4112403 initial commit on master

现在添加——first-parent:

$ git log --all --oneline --decorate --graph --first-parent

* a9546a2 (HEAD -> master, origin/master, origin/HEAD) merge from topic back to master
| * 648ca35 (origin/topic) merging master onto topic
| * 132ee2a first commit on topic branch
* | e7c863d commit on master after master was merged to topic
* | 37ad159 post-branch commit on master
|/  
* 6aafd7f second commit on master before branching
* 4112403 initial commit on master

这样就简单多了!

注意，如果repo有很多分支，你会想要指定两个分支进行比较，而不是使用——all:

$ git log --decorate --oneline --graph --first-parent master origin/topic