use parse_object_or_die instead of die("bad object")

Some call-sites do:

  o = parse_object(sha1);
  if (!o)
	  die("bad object %s", some_name);

We can now handle that as a one-liner, and get more
consistent output.

In the third case of this patch, it looks like we are losing
information, as the existing message also outputs the sha1
hex; however, parse_object will already have written a more
specific complaint about the sha1, so there is no point in
repeating it here.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/prune.c

@@ -149,9 +149,7 @@ int cmd_prune(int argc, const char **argv, const char *prefix)
 		const char *name = *argv++;
 
 		if (!get_sha1(name, sha1)) {
-			struct object *object = parse_object(sha1);
-			if (!object)
-				die("bad object: %s", name);
+			struct object *object = parse_object_or_die(sha1, name);
 			add_pending_object(&revs, object, "");
 		}
 		else