For `doc_id` returns a list of:
            [0] the repo name and,
            [1] 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_repo_and_path_fragment(self, doc_id):
        """For `doc_id` returns a list of:
            [0] the repo name and,
            [1] 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.
        """
        shard = self.get_shard(doc_id)
        return shard.name, shard.get_rel_path_fragment(doc_id)