Add has_extension()

The little helper has_extension() documents through its name what we are
trying to do and makes sure we don't forget the underrun check.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>

index-pack.c

@@ -447,7 +447,7 @@ int main(int argc, char **argv)
 		usage(index_pack_usage);
 	if (!index_name) {
 		int len = strlen(pack_name);
-		if (len < 5 || strcmp(pack_name + len - 5, ".pack"))
+		if (!has_extension(pack_name, len, ".pack"))
 			die("packfile name '%s' does not end with '.pack'",
 			    pack_name);
 		index_name_buf = xmalloc(len);