completion: add branch options --contains --merged --no-merged

Signed-off-by: Eric Raible <raible@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -546,7 +546,7 @@ _git_branch ()
 	--*)
 		__gitcomp "
 			--color --no-color --verbose --abbrev= --no-abbrev
-			--track --no-track
+			--track --no-track --contains --merged --no-merged
 			"
 		;;
 	*)