Similar to --author/--committer which filters commits by author and
committer header fields. --grep-reflog adds a fake "reflog" header to
commit and a grep filter to search on that line.

All rules to --author/--committer apply except no timestamp stripping.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>