Added another migration-executor test to avoid regressions.

        should_be_rolled_back = [b2_impl, a4_impl, a2_impl, a3_impl]

        exp = [(m, True) for m in should_be_rolled_back]

        self.assertEqual(plan, exp)

    def test_backwards_nothing_to_do(self):

        """

        If the current state satisfies the given target, do nothing.

        a: 1 <--- 2

        b:    \- 1

        c:     \- 1

        If a1 is applied already and a2 is not, and we're asked to migrate to

        a1, don't apply or unapply b1 or c1, regardless of their current state.

        """

        a1_impl = FakeMigration('a1')

        a1 = ('a', '1')

        a2_impl = FakeMigration('a2')

        a2 = ('a', '2')

        b1_impl = FakeMigration('b1')

        b1 = ('b', '1')

        c1_impl = FakeMigration('c1')

        c1 = ('c', '1')

        graph = MigrationGraph()

        graph.add_node(a1, a1_impl)

        graph.add_node(a2, a2_impl)

        graph.add_node(b1, b1_impl)

        graph.add_node(c1, c1_impl)

        graph.add_dependency(None, a2, a1)

        graph.add_dependency(None, b1, a1)

        graph.add_dependency(None, c1, a1)

        executor = MigrationExecutor(None)

        executor.loader = FakeLoader(graph, {a1, b1})

        plan = executor.migration_plan({a1})

        self.assertEqual(plan, [])