R1:65c1c758ed26

Use extended policies in Differential diffs

Summary:
Fixes T9648. Diffs currently use `return $this->getRevision()->getViewPolicy();` to inherit their revision's view policy.

After the introduction of object policies, this is wrong for policies like "Subscribers", because it means "Subscribers to this object, the diff". Since Diffs have no subscribers, this always fails.

Instead, use extended policies so that the object policy evaluates in the context of the correct object (the revision).

Test Plan…