Prefetch specific model instances

This reduces the number of queries performed during delete, slightly.

Was 191 before this change

wagtail/actions/delete_page.py

@@ -33,8 +33,8 @@ class DeletePageAction:
         # works around a bug in treebeard <= 3.0 where calling SpecificPage.delete() fails to delete
         # child pages that are not instances of SpecificPage
         if type(page) is Page:
-            for child in page.get_descendants():
-                self.log_deletion(child.specific)
+            for child in page.get_descendants().specific():
+                self.log_deletion(child)
             self.log_deletion(page.specific)
 
             # this is a Page instance, so carry on as we were

wagtail/tests/test_audit_log.py

@@ -285,7 +285,9 @@ class TestAuditLog(TestCase):
         )
 
         # check deleting a parent page logs descendent deletion
+        with self.assertNumQueries(189):
             self.home_page.delete()
+
         self.assertEqual(
             PageLogEntry.objects.filter(action="wagtail.delete").count(), 4
         )