use parse_commit_or_die instead of custom message

Many calls to parse_commit detect errors and die. In some
cases, the custom error messages are more useful than what
parse_commit_or_die could produce, because they give some
context, like which ref the commit came from. Some, however,
just say "invalid commit". Let's convert the latter to use
parse_commit_or_die; its message is slightly more informative,
and it makes the error more consistent throughout git.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

shallow.c

@@ -89,8 +89,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 				cur_depth = *(int *)commit->util;
 			}
 		}
-		if (parse_commit(commit))
-			die("invalid commit");
+		parse_commit_or_die(commit);
 		cur_depth++;
 		if (cur_depth >= depth) {
 			commit_list_insert(commit, &result);

upload-pack.c

@@ -694,8 +694,7 @@ static void receive_needs(void)
 				/* make sure the real parents are parsed */
 				unregister_shallow(object->sha1);
 				object->parsed = 0;
-				if (parse_commit((struct commit *)object))
-					die("invalid commit");
+				parse_commit_or_die((struct commit *)object);
 				parents = ((struct commit *)object)->parents;
 				while (parents) {
 					add_object_array(&parents->item->object,