Finding a branch point with Git?

Question

I have a repository with branches master and A and lots of merge activity between the two. How can I find the commit in my repository when branch A was created based on mast

Answer 1

Not quite a solution to the question but I thought it was worth noting the the approach I use when I have a long-living branch:

At the same time I create the branch, I also create a tag with the same name but with an -init suffix, for example feature-branch and feature-branch-init.

(It is kind of bizarre that this is such a hard question to answer!)