ASSUMES the caller holds the _doc_counter_lock !
        Checks the current int value of the next ottid, reserves a block of
        {how_many} ids, advances the counter to the next available value,
        stores the counter in a file in case the server is restarted.
        Checks out master branch as a side effect.

def _mint_new_ott_ids(self, how_many=1):
        """ ASSUMES the caller holds the _doc_counter_lock !
        Checks the current int value of the next ottid, reserves a block of
        {how_many} ids, advances the counter to the next available value,
        stores the counter in a file in case the server is restarted.
        Checks out master branch as a side effect."""
        first_minted_id = self._next_ott_id
        self._next_ott_id = first_minted_id + how_many
        content = u'{"next_ott_id": %d}\n' % self._next_ott_id
        # The content is JSON, but we hand-rolled the string above
        #       so that we can use it as a commit_msg
        self._write_master_branch_resource(content,
                                           self._id_minting_file,
                                           commit_msg=content,
                                           is_json=False)
        last_minted_id = self._next_ott_id - 1
        return first_minted_id, last_minted_id