Fix proxy model Query.remove_inherited_models()

Fixed #18248 -- proxy models were added to included_inherited_models
in sql.query.Query. The variable is meant to be used for multitable
inheritance only. This mistake caused problems in situations where
proxy model's query was reused.

tests/regressiontests/queries/tests.py

@@ -19,7 +19,7 @@ from .models import (Annotation, Article, Author, Celebrity, Child, Cover,
     ManagedModel, Member, NamedCategory, Note, Number, Plaything, PointerA,
     Ranking, Related, Report, ReservedName, Tag, TvChef, Valid, X, Food, Eaten,
     Node, ObjectA, ObjectB, ObjectC, CategoryItem, SimpleCategory,
-    SpecialCategory, OneToOneCategory, NullableName)
+    SpecialCategory, OneToOneCategory, NullableName, ProxyCategory)
 
 
 class BaseQuerysetTest(TestCase):
@@ -1952,3 +1952,15 @@ class EmptyStringsAsNullTest(TestCase):
             DumbCategory.objects.exclude(namedcategory__name__in=['nonexisting']),
             [self.nc.pk], attrgetter('pk')
         )
+
+class ProxyQueryCleanupTest(TestCase):
+    def test_evaluated_proxy_count(self):
+        """
+        Test that generating the query string doesn't alter the query's state
+        in irreversible ways. Refs #18248.
+        """
+        ProxyCategory.objects.create()
+        qs = ProxyCategory.objects.all()
+        self.assertEqual(qs.count(), 1)
+        str(qs.query)
+        self.assertEqual(qs.count(), 1)