git-oldest-common-ancestor

#!/usr/bin/env bash
#
# Outputs the SHA of the oldest common ancestor of two branches.
#
# The first argument defaults to `master` and the second argument defaults to `HEAD`.
#
# Original source: https://github.com/lee-dohm/dotfiles/blob/main/bin/git-oldest-ancestor

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

exec git rev-parse --short "$ancestor"