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 master?

My repository basically looks like this:

-- X -- A -- B -- C -- D -- F  (master) 
          \     /   \     /
           \   /     \   /
             G -- H -- I -- J  (branch A)

I'm looking for revision A, which is not what git merge-base (--all) finds.

Answer 1

The following implements git equivalent of svn log --stop-on-copy and can also be used to find branch origin.

Approach

1. Get head for all branches
2. collect mergeBase for target branch each other branch
3. git.log and iterate
4. Stop at first commit that appears in the mergeBase list

Like all rivers run to the sea, all branches run to master and therefore we find merge-base between seemingly unrelated branches. As we walk back from branch head through ancestors, we can stop at the first potential merge base since in theory it should be origin point of this branch.

Notes

• I haven't tried this approach where sibling and cousin branches merged between each other.
• I know there must be a better solution.

details: https://stackoverflow.com/a/35353202/9950

Answer 2

You may be looking for git merge-base:

git merge-base finds best common ancestor(s) between two commits to use in a three-way merge. One common ancestor is better than another common ancestor if the latter is an ancestor of the former. A common ancestor that does not have any better common ancestor is a best common ancestor, i.e. a merge base. Note that there can be more than one merge base for a pair of commits.

Answer 3

I've used git rev-list for this sort of thing. For example, (note the 3 dots)

$ git rev-list --boundary branch-a...master | grep "^-" | cut -c2-

will spit out the branch point. Now, it's not perfect; since you've merged master into branch A a couple of times, that'll split out a couple possible branch points (basically, the original branch point and then each point at which you merged master into branch A). However, it should at least narrow down the possibilities.

I've added that command to my aliases in ~/.gitconfig as:

[alias]
    diverges = !sh -c 'git rev-list --boundary $1...$2 | grep "^-" | cut -c2-'

so I can call it as:

$ git diverges branch-a master