How to get commit where merged branch forked from

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.

I think the following does what you are looking for:

git merge-base --fork-point master

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 !