Works around a sync-time performance regression on PG12

closes pulp#4591

Author: dralley

CHANGES/4591.bugfix

@@ -0,0 +1 @@
+Resolved a sync-time performance regression.

pulpcore/app/models/repository.py

@@ -850,7 +850,9 @@ def add_content(self, content):
             raise ResourceImmutableError(self)
 
         repo_content = []
-        to_add = set(content.exclude(pk__in=self.content).values_list("pk", flat=True))
+        to_add = set(content.values_list("pk", flat=True)) - set(
+            self.content.values_list("pk", flat=True)
+        )
 
         # Normalize representation if content has already been removed in this version and
         # is re-added: Undo removal by setting version_removed to None.