For `doc_id` returns the path from the
        repo to the doc file. This is useful because
        (if you know the remote), it lets you construct the full path.

def get_rel_path_fragment(self, doc_id):
        """For `doc_id` returns the path from the
        repo to the doc file. This is useful because
        (if you know the remote), it lets you construct the full path.
        """
        with self._index_lock:
            r = self._doc_index[doc_id]
        fp = r[-1]
        return fp[(len(self.path) + 1):]