mfc-candidates: fix output order with incorrect commit dates

Previously we obtained the list of MFC candidate hashes (i.e., those
commits only in the mfc-from branch), removed the already-merged
commits based on "cherry picked from" commit message entries, and then
printed the list sorted by commit time. This is usually the correct
order, but it is not when commits have non-monotonically-increasing
commit times. This is unfortunately the case for a number of commits in
our tree. This made it difficult to merge some groups of dependent
commits in the correct order.

Instead, leave the from-list in git rev-parse order, and move the
matching logic to a new lua script.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44808