How to get commit where merged branch forked from

Question

o---1---o---o---o---o     master
     \         /
      o---o---2           feature

Is it possible to get commit 1 without using reflog ? Using git 2.4.2

I tried:

git merge-base master feature
git merge-base feature master
returns: commit 2 
git merge-base --fork-point master feature
git merge-base --fork-point feature master
returns nothing ( exit code 1 )

Not duplicates:

• How to get a branch into a forked repo from the original repo in github
• git how to find commit hash where branch originated from
• How to get information where branch starts?

Answer 1

Found my answer as a subset from this question: ( backporting was in fact also my usecase ! ) https://stackoverflow.com/a/16718891/867294

[alias]
    oldest-ancestor = !bash -c 'diff -u <(git rev-list --first-parent "${1:-master}") <(git rev-list --first-parent "${2:-HEAD}") | sed -ne \"s/^ //p\" | head -1' -

I am somewhat baffled that this is not part of git as default. You would at least expect that this would be the behavior of the --fork-point option for merge-base. If anyone knows a better/default alternative. Please put it in the comments or as a separate answer !

Answer 2

As you noticed, git merge-base will not help you. The reason for this is that feature is part of master’s history and it could be fast-forwarded to master.

If this is your specific setup though, that you have merged feature into master, and the feature branch still points to the commit before the merge, then you can just get all the commits that are included in master but not included in feature and take the last one; the merge base you are looking for is its parent. You can do that by specifying the range feature..master:

git show $(git rev-list feature..master | tail -1)~1

This will show you the merge base commit “1” in your graph.