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

The problem appears to be to find the most recent, single-commit cut between both branches on one side, and the earliest common ancestor on the other (probably the initial commit of the repo). This matches my intuition of what the "branching off" point is.

That in mind, this is not at all easy to compute with normal git shell commands, since git rev-list -- our most powerful tool -- doesn't let us restrict the path by which a commit is reached. The closest we have is git rev-list --boundary, which can give us a set of all the commits that "blocked our way". (Note: git rev-list --ancestry-path is interesting but I don't how to make it useful here.)

Here is the script: https://gist.github.com/abortz/d464c88923c520b79e3d. It's relatively simple, but due to a loop it's complicated enough to warrant a gist.

Note that most other solutions proposed here can't possibly work in all situations for a simple reason: git rev-list --first-parent isn't reliable in linearizing history because there can be merges with either ordering.

git rev-list --topo-order, on the other hand, is very useful -- for walking commits in topographic order -- but doing diffs is brittle: there are multiple possible topographic orderings for a given graph, so you are depending on a certain stability of the orderings. That said, strongk7's solution probably works damn well most of the time. However it's slower that mine as a result of having to walk the entire history of the repo... twice. :-)