t7810-grep: test --all-match with multiple --grep and --author options

The code used to have a bug that ignores "--all-match", that requires
all "--grep" to have matched, when "--author" or "--committer" was used.

Make sure the bug will not be reintroduced.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>