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/grep.c

@@ -898,9 +898,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		unsigned char sha1[20];
 		/* Is it a rev? */
 		if (!get_sha1(arg, sha1)) {
-			struct object *object = parse_object(sha1);
-			if (!object)
-				die(_("bad object %s"), arg);
+			struct object *object = parse_object_or_die(sha1, arg);
 			add_object_array(object, arg, &list);
 			continue;
 		}

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

reachable.c

@@ -152,11 +152,9 @@ static int add_one_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
 
 static int add_one_ref(const char *path, const unsigned char *sha1, int flag, void *cb_data)
 {
-	struct object *object = parse_object(sha1);
+	struct object *object = parse_object_or_die(sha1, path);
 	struct rev_info *revs = (struct rev_info *)cb_data;
 
-	if (!object)
-		die("bad object ref: %s:%s", path, sha1_to_hex(sha1));
 	add_pending_object(revs, object, "");
 
 	return 0;