completion: add --graph to log command completion

Signed-off-by: NDan McGee <dpmcgee@gmail.com>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -761,6 +761,7 @@ _git_log ()
 			--pretty= --name-status --name-only --raw
 			--not --all
 			--left-right --cherry-pick
+			--graph
 			"
 		return
 		;;