GIT find commit where branched off

oldest-ancestor-alias.sh

#!/bin/bash

# "master" is the name of your main branch you'll be comparing to, or you can pass the "parent" branch as an argument
# it compares against the current branch by default, but you can pass a second argument to specify a child branch

# this creates a permanent alias so you can use `git oldest-ancestor` to get the oldest ancestor of the current branch at any time

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'\'' -'

oldest-ancestor.sh

#!/bin/bash

# $1 is the main ancestor branch to compare to (defaults to `master`)
# $2 is the child branch to find the ancestor of (defaults to current branch)

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