git: how do I find the common ancestor of two branches… that have already been merged

问题

In order to find the common ancestor of 2 git branches, one needs to do:

git merge-base branch another_branch

Okay. But... what if both branches have already been merged? When I use merge-base in that case, the commit I get is the latest one before the merge, and I would like to know the common ancestor from which both branches came out "before the merge".

In other words:

Z - A - B - C - D - E - F
     \          /
      G - H - I 

HEAD is at F. If I do a git merge-base here, the commit I get is I, and the one I am looking for is A, or perhaps Z. Is there any git instruction to find it?

回答1:

If you do

git merge-base C I

you get the commit A you want. What are C and I? They are the parent commits of your merge commit D.

Hoy do you get them?

git log --pretty=%P -n 1 D 

gives you two strings. This two strings are the commit C and I you need.

If you want to do it all together you can write

git merge-base $(git log --pretty=%P -n 1 MERGE_COMMIT)

where you have to replace MERGE_COMMIT with the hash of your merge commit D

Edit: As @poke stated it is simpler with

git merge-base MERGE_COMMIT^ MERGE_COMMIT^2 

回答2:

I usually need to find the ancestor between the tip of my branch, HEAD and the remote main development branch, say origin/dev. Here is what I use :

git log --reverse --boundary --format=%h HEAD ^origin/dev | head -1

It essentially says:

• Take all the ancestors except the ones common with origin/dev
• Include the boundary (the forking point)
• Reverse the order so the forking point is the first in the list
• Log to a format where only the SHA appears
• Take the first one in this list

来源：https://stackoverflow.com/questions/43712418/git-how-do-i-find-the-common-ancestor-of-two-branches-that-have-already-been