toolkit: Add closest-branch command

[jerry-skydio]

This can be used to find where branches
diverge.

Topic: closest-b
Reviewers: brian-k

[jerry-skydio]

Reviews in this chain:
└#62 toolkit: Add closest-branch command

[jerry-skydio]

#	head	base	diff	date	summary
0	1f939872	0e63b494	diff	Sep 21 7:09 PM	3 files changed, 29 insertions(+), 7 deletions(-)
1	46ac6352	5c6d303f	diff	Sep 22 11:21 AM	2 files changed, 10 insertions(+), 20 deletions(-)
2	02a03912	7d7e58f3	rebase	Sep 22 11:22 AM	0 files changed
3	08e98590	a8ae3518	diff	Sep 22 11:29 AM	2 files changed, 10 insertions(+), 8 deletions(-)
4	12481615	a8ae3518	diff	Sep 22 11:30 AM	1 file changed, 1 insertion(+), 1 deletion(-)
5	469c7945	a8ae3518	diff	Sep 22 11:35 AM	1 file changed, 2 insertions(+), 2 deletions(-)
6	d5a4e510	a8ae3518	diff	Sep 23 12:37 PM	1 file changed, 1 insertion(+), 1 deletion(-)