contrib/completion: --no-index option to git diff

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

contrib/completion/git-completion.bash

@@ -1125,7 +1125,7 @@ _git_diff ()
 	case "$cur" in
 	--*)
 		__gitcomp "--cached --staged --pickaxe-all --pickaxe-regex
-			--base --ours --theirs
+			--base --ours --theirs --no-index
 			$__git_diff_common_options
 			"
 		return