completion: support format-patch's --cover-letter option

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

contrib/completion/git-completion.bash

@@ -648,6 +648,7 @@ _git_format_patch ()
 			--in-reply-to=
 			--full-index --binary
 			--not --all
+			--cover-letter
 			"
 		return
 		;;