Fixed #29159 -- Made ModelChoiceIterator reuse QuerySet result cache.

When __len__() is called (e.g. when casting to list or tuple), the
QuerySet is evaluated and the result cache populated. iterator()
shouldn't be called on the QuerySet after that, as it would reset the
result cache and trigger a second query.

tests/model_forms/test_modelchoicefield.py

@@ -257,6 +257,17 @@ class ModelChoiceFieldTests(TestCase):
             (self.c3.pk, 'Third'),
         ])
 
+    def test_queryset_result_cache_is_reused(self):
+        f = forms.ModelChoiceField(Category.objects.all())
+        with self.assertNumQueries(1):
+            # list() calls __len__() and __iter__(); no duplicate queries.
+            self.assertEqual(list(f.choices), [
+                ('', '---------'),
+                (self.c1.pk, 'Entertainment'),
+                (self.c2.pk, 'A test'),
+                (self.c3.pk, 'Third'),
+            ])
+
     def test_num_queries(self):
         """
         Widgets that render multiple subwidgets shouldn't make more than one