Add a tool to find MFC candidates

usage: mfc-candidates.sh [-ah] [-f from_branch] [-t to_branch]

[-u user] [-X exclude_file] [path ...]

This tool compares git commit hashes that are in from_branch and not in
to_branch with hashes listed in "cherry picked from" lines, finding
commits that may be MFC candidates. By default it searches for commits
by ${USER} but that can be overridden with -u for a specific user or -a
for all.

gonzo@'s MFC tracker is a web application that presents this sort of
information in a more convenient way, and unlike this script it tracks
"MFC After" status and timing. However, it is sometimes useful to be
able to perform an offline search against a local repo.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34208