completion: complete --cherry-mark for git log

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

contrib/completion/git-completion.bash

@@ -1462,6 +1462,7 @@ _git_log ()
 			--relative-date --date=
 			--pretty= --format= --oneline
 			--show-signature
+			--cherry-mark
 			--cherry-pick
 			--graph
 			--decorate --decorate=