tools/git: Add git-arc

This is a handy script for creating and updating Differential revisions
from git commits. It tries to avoid forcing the user to manage their
git tree in any particular way, but makes two major assumptions:

• there is a one-to-one mapping between git commits and Differential revisions,
• the title of a Differential revision is the same as the summary line of the corresponding commit.

A verbose description of the script's functionality is provided in its
usage message, which should probably be converted to a man page.

A description of workflows using git-arc is here:
https://lists.freebsd.org/pipermail/freebsd-hackers/2021-January/056979.html

There are some loose ends but this is functional enough to be useful.

Discussed with: jhb
Differential Revision: https://reviews.freebsd.org/D28334