mfc-candidates: search by committer only, not author

When both --author and --committer are specified, git log requires
both to match. Search only by committer, as it is typically the FreeBSD
committer who will perform the MFC.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41964

(cherry picked from commit 61f8df6a13a357e1d45c22a2016b87491f10de87)