Fixed #30856 -- Combined fast-delete queries by model during cascade deletion.

Reduced the number of queries required when performing cascade deletion
for a model referenced multiple time by another one by performing an
union of reference lookups.

tests/delete/tests.py

@@ -582,3 +582,11 @@ class FastDeleteTests(TestCase):
                 User.objects.filter(avatar__desc='missing').delete(),
                 (0, {'delete.User': 0})
             )
+
+    def test_fast_delete_combined_relationships(self):
+        # The cascading fast-delete of SecondReferrer should be combined
+        # in a single DELETE WHERE referrer_id OR unique_field.
+        origin = Origin.objects.create()
+        referer = Referrer.objects.create(origin=origin, unique_field=42)
+        with self.assertNumQueries(2):
+            referer.delete()