as self.parents[0] - if it is not, errors may occur.
          iter_changes: An iter_changes iterator with the changes to apply
            to basis_revision_id. The iterator must not include any items with
            a current kind of None - missing items must be either filtered out
            or errored-on beefore record_iter_changes sees the item.
        Returns: A generator of (relpath, fs_hash) tuples for use with
            tree._observed_sha1.
        )