grep: do not use --index in the short usage output

Utilize the PARSE_OPT_NEGHELP option to show --no-index in the usage
generated by parse-options.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -762,8 +762,9 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	struct option options[] = {
 		OPT_BOOLEAN(0, "cached", &cached,
 			"search in index instead of in the work tree"),
-		OPT_BOOLEAN(0, "index", &use_index,
-			"--no-index finds in contents not managed by git"),
+		{ OPTION_BOOLEAN, 0, "index", &use_index, NULL,
+			"finds in contents not managed by git",
+			PARSE_OPT_NOARG | PARSE_OPT_NEGHELP },
 		OPT_GROUP(""),
 		OPT_BOOLEAN('v', "invert-match", &opt.invert,
 			"show non-matching lines"),