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

Resolve it as a .git-config line:

[alias]

    oldest-ancestor = !zsh -c 'diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent "${1:-master}") <(git rev-list --first-parent "${2:-HEAD}") | head -1' -

Also can be done from the shell:

git config --global alias.oldest-ancestor '!zsh -c '\''diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent "${1:-master}") <(git rev-list --first-parent "${2:-HEAD}") | head -1'\'' -'