'If the last ``Snapshot`` in the ``list``\ s passed to
``_latest_common_snapshot`` is the same, it is returned.'

def test_last_snapshot_common(self):

    a = Snapshot(name='a')
    b = Snapshot(name='b')
    c = Snapshot(name='c')
    self.assertEqual(a, _latest_common_snapshot([b, a], [c, a]))
