Code to find the first point a git branch started to diverge

branch-point

#!/bin/bash

# actual code

find_merge ()
{
	local selection extra
	test "$2" && extra=" into $2"
	git rev-list --min-parents=2 --grep="Merge branch '$1'$extra" --topo-order ${3:---all} | tail -1
}

branch_point ()
{
	local first_merge second_merge merge
	first_merge=$(find_merge $1 "" "$1 $2")
	second_merge=$(find_merge $2 $1 $first_merge)
	merge=${second_merge:-$first_merge}

	# echo "merge: $merge" > /dev/stderr
	if [ "$merge" ]; then
		git merge-base $merge^1 $merge^2
	else
		git merge-base $1 $2
	fi
}

# testing

do_repo ()
{
	repo=$1
	git init -q $1 && cd $1
}

do_commit ()
{
	local branch base
	if [ "$2" ]; then
		test "$3" && base="refs/commits/$3" || base="$(git show-ref -s --heads $2)"
		git checkout -q -B "$2" "${base:-HEAD}"
	fi
	branch=$(git symbolic-ref --short HEAD)
	git commit --allow-empty -q -m "$1 - $branch"
	git update-ref refs/commits/$1 HEAD
}

do_merge ()
{
	local branch extra
	test "$3" && git checkout -q $3
	branch=$(git symbolic-ref --short HEAD)
	test $branch != 'master' && extra=" into $branch"
	git merge -q $2 -m "$1 - Merge branch '$2'$extra"
	git update-ref refs/commits/$1 HEAD
}

show_repo ()
{
	echo "== $repo =="
	git log --oneline --decorate --graph --all --topo-order
}

do_test ()
{
	c=$(branch_point $1 $2)
	r=$(git name-rev --name-only --refs='refs/commits/*' $c)
	s=${r#commits/}
	echo "result: $s"
}

rm -rf test*

# X (master) (topic)
(
	do_repo 'test1'
	do_commit 'X'
	git checkout -q -b topic master
	show_repo
	do_test 'topic' 'master'
)

# X (master)
#  \
#   A (topic)
(
	do_repo 'test2'
	do_commit 'X'
	do_commit 'A' 'topic'
	show_repo
	do_test 'topic' 'master'
)

# X - B (master)
#  \
#   A (topic)
(
	do_repo 'test3'
	do_commit 'X'
	do_commit 'A' 'topic'
	do_commit 'B' 'master'
	show_repo
	do_test 'topic' 'master'
)

# X - B - C (master)
#  \     /
#   --- A (topic)
(
	do_repo 'test4'
	do_commit 'X'
	do_commit 'A' 'topic'
	do_commit 'B' 'master'
	do_merge 'C' 'topic'
	show_repo
	do_test 'topic' 'master'
)

# X --- B (master)
#  \    \
#   A - C (topic)
(
	do_repo 'test5'
	do_commit 'X'
	do_commit 'A' 'topic'
	do_commit 'B' 'master'
	do_merge 'C' 'master' 'topic'
	show_repo
	do_test 'topic' 'master'
)

# X - B - C - E (master)
#  \     /   /
#   --- A - D (topic)
(
	do_repo 'test6'
	do_commit 'X'
	do_commit 'A' 'topic'
	do_commit 'B' 'master'
	do_merge 'C' 'topic'
	do_commit 'D' 'topic'
	do_merge 'E' 'topic' 'master'
	show_repo
	do_test 'topic' 'master'
)

# X --- B - D - F (master)
#  \    \      /
#   A - C --- E (topic)
(
	do_repo 'test7'
	do_commit 'X'
	do_commit 'A' 'topic'
	do_commit 'B' 'master'
	do_merge 'C' 'master' 'topic'
	do_commit 'D' 'master'
	do_commit 'E' 'topic'
	do_merge 'F' 'topic' 'master'
	show_repo
	do_test 'topic' 'master'
)

# X - A ----- F (master)
#  \         /
#   B - C - E (topic)
#    \     /
#     D - + (test)
(
	do_repo 'test8'
	do_commit 'X'
	do_commit 'A'
	do_commit 'B' 'topic' 'X'
	do_commit 'C'
	do_commit 'D' 'test' 'B'
	do_merge 'E' 'test' 'topic'
	do_merge 'F' 'topic' 'master'
	show_repo
	do_test 'topic' 'master'
)

# X - A ----- F (master)
#  \         /
#   B - C - + (topic)
#    \   \
#     D - E (test)
(
	do_repo 'test9'
	do_commit 'X'
	do_commit 'A'
	do_commit 'B' 'topic' 'X'
	do_commit 'C'
	do_commit 'D' 'test' 'B'
	do_merge 'E' 'topic' 'test'
	do_merge 'F' 'topic' 'master'
	show_repo
	do_test 'topic' 'master'
)

# X - A ---- E - G (master)
#  \        /   /
#  + - B - D   / (fix)
#   \     /   /
#    C - + - F (topic)
(
	do_repo 'test10'
	do_commit 'X'
	do_commit 'A'
	do_commit 'B' 'fix' 'X'
	do_commit 'C' 'topic' 'X'
	do_merge 'D' 'topic' 'fix'
	do_merge 'E' 'fix' 'master'
	do_commit 'F' 'topic'
	do_merge 'G' 'topic' 'master'
	show_repo
	do_test 'topic' 'master'
)