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

In general, this is not possible. In a branch history a branch-and-merge before a named branch was branched off and an intermediate branch of two named branches look the same.

In git, branches are just the current names of the tips of sections of history. They don't really have a strong identity.

This isn't usually a big issue as the merge-base (see Greg Hewgill's answer) of two commits is usually much more useful, giving the most recent commit which the two branches shared.

A solution relying on the order of parents of a commit obviously won't work in situations where a branch has been fully integrated at some point in the branch's history.

git commit --allow-empty -m root # actual branch commit
git checkout -b branch_A
git commit --allow-empty -m  "branch_A commit"
git checkout master
git commit --allow-empty -m "More work on master"
git merge -m "Merge branch_A into master" branch_A # identified as branch point
git checkout branch_A
git merge --ff-only master
git commit --allow-empty -m "More work on branch_A"
git checkout master
git commit --allow-empty -m "More work on master"

This technique also falls down if an integration merge has been made with the parents reversed (e.g. a temporary branch was used to perform a test merge into master and then fast-forwarded into the feature branch to build on further).

git commit --allow-empty -m root # actual branch point
git checkout -b branch_A
git commit --allow-empty -m  "branch_A commit"
git checkout master
git commit --allow-empty -m "More work on master"
git merge -m "Merge branch_A into master" branch_A # identified as branch point
git checkout branch_A
git commit --allow-empty -m "More work on branch_A"

git checkout -b tmp-branch master
git merge -m "Merge branch_A into tmp-branch (master copy)" branch_A
git checkout branch_A
git merge --ff-only tmp-branch
git branch -d tmp-branch

git checkout master
git commit --allow-empty -m "More work on master"