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:
  - Create a revision.
  - Subscribe `alice` to it.
  - Set view policy to "Subscribers".
  - View revision as `alice`.
  - Before patch: nonsense fatal about missing diff because of policy error.
  - After patch: `alice` can see the revision.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9648

Differential Revision: https://secure.phabricator.com/D17123