completion: also complete git-log's --left-right and --cherry-pick option

Both --left-right and --cherry-pick are particularly long to type, so
help the user there.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -641,6 +641,7 @@ _git_log ()
 			--all-match
 			--pretty= --name-status --name-only --raw
 			--not --all
+			--left-right --cherry-pick
 			"
 		return
 		;;