checkout: convert post_checkout_hook() to struct object_id

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

builtin/checkout.c

@@ -56,8 +56,8 @@ static int post_checkout_hook(struct commit *old, struct commit *new,
 			      int changed)
 {
 	return run_hook_le(NULL, "post-checkout",
-			   sha1_to_hex(old ? old->object.oid.hash : null_sha1),
-			   sha1_to_hex(new ? new->object.oid.hash : null_sha1),
+			   oid_to_hex(old ? &old->object.oid : &null_oid),
+			   oid_to_hex(new ? &new->object.oid : &null_oid),
 			   changed ? "1" : "0", NULL);
 	/* "new" can be NULL when checking out from the index before
 	   a commit exists. */