Checks out master branch as a side effect

def _mint_new_study_id(self):
        """Checks out master branch as a side effect"""
        # studies created by the OpenTree API start with ot_,
        # so they don't conflict with new study id's from other sources
        with self._doc_counter_lock:
            c = self._advance_new_study_id()
        # @TODO. This form of incrementing assumes that
        #   this codebase is the only service minting
        #   new study IDs!
        return "{p}{c:d}".format(p=self._new_study_prefix, c=c)