'Add or modify graftpoints
:param updated_graftpoints: Dict of commit shas to list of parent shas'

def _add_graftpoints(self, updated_graftpoints):

    for (commit, parents) in updated_graftpoints.items():
        for sha in ([commit] + parents):
            check_hexsha(sha, 'Invalid    graftpoint')
    self._graftpoints.update(updated_graftpoints)
