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

You could use the following command to return the oldest commit in branch_a, which is not reachable from master:

git rev-list branch_a ^master | tail -1

Perhaps with an additional sanity check that the parent of that commit is actually reachable from master...