dir: point treat_leading_path() warning to the right place

Commit 777b420347 (dir: synchronize treat_leading_path() and
read_directory_recursive(), 2019-12-19) tried to add two warning
comments in those functions, pointing at each other. But the one in
treat_leading_path() just points at itself.

Let's fix that. Since the comment also redirects the reader for more
details to "the commit that added this warning", and since we're now
modifying the warning (creating a new commit without those details),
let's mention the actual commit id.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -2310,9 +2310,9 @@ static int treat_leading_path(struct dir_struct *dir,
 	 * WARNING WARNING WARNING:
 	 *
 	 * Any updates to the traversal logic here may need corresponding
-	 * updates in treat_leading_path().  See the commit message for the
-	 * commit adding this warning as well as the commit preceding it
-	 * for details.
+	 * updates in read_directory_recursive().  See 777b420347 (dir:
+	 * synchronize treat_leading_path() and read_directory_recursive(),
+	 * 2019-12-19) and its parent commit for details.
 	 */
 
 	struct strbuf sb = STRBUF_INIT;