Allow logged-out access to individual diffs (vs revisions)

Summary: Fixes T5303. Individual diffs can have public access policies.

Test Plan: Viewed a public diff while logged out.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5303

Differential Revision: https://secure.phabricator.com/D9452