[1.8.x] Fixed #25852 -- Made sure AlterModelManager forces a reload of its model state.

Thanks to Geoffrey Sechter and the Django NYC group for the report and Markus
for the review.

Backport of c4e372aaf4 from master

django/db/migrations/operations/models.py

@@ -563,6 +563,7 @@ class AlterModelManagers(Operation):
     def state_forwards(self, app_label, state):
         model_state = state.models[app_label, self.name_lower]
         model_state.managers = list(self.managers)
+        state.reload_model(app_label, self.name_lower)
 
     def database_forwards(self, app_label, schema_editor, from_state, to_state):
         pass

docs/releases/1.8.8.txt

@@ -34,3 +34,6 @@ Bugfixes
 * Fixed missing ``varchar/text_pattern_ops`` index on ``CharField`` and
   ``TextField`` respectively when using ``AlterField`` on PostgreSQL
   (:ticket:`25412`).
+
+* Fixed a state bug when using an ``AlterModelManagers`` operation
+  (:ticket:`25852`).

tests/migrations/test_operations.py

@@ -1378,6 +1378,11 @@ class OperationTests(OperationTestBase):
         self.assertEqual(managers[2][0], "food_mgr_kwargs")
         self.assertIsInstance(managers[2][1], FoodManager)
         self.assertEqual(managers[2][1].args, ("x", "y", 3, 4))
+        rendered_state = new_state.apps
+        model = rendered_state.get_model('test_almoma', 'pony')
+        self.assertIsInstance(model.food_qs, models.Manager)
+        self.assertIsInstance(model.food_mgr, FoodManager)
+        self.assertIsInstance(model.food_mgr_kwargs, FoodManager)
 
     def test_alter_model_managers_emptying(self):
         """