One common request we've had over the years is to mirror repository

access control into Review Board. This is complicated by a few factors.

Review Board has always used a single account to access the remote

repository, and it caches aggressively to provide reasonable interactive

performance, so we can't just rely on the repository access directly. In

addition, it's not uncommon for repository users/usernames to not match

up with Review Board users/usernames (especially when repositories are

hosted on SaaS services).
In order to make this possible, we're adding a new extension hook,

FileDiffACLsHook. This hook is invoked when a user attempts to access

a review request, and is run for each DiffSet/FileDiff. The result

of this is cached per DiffSet to keep reasonable performance overall.

People can then create an extension implementing this hook to do

whatever access control they want, whether that's querying the

repository or referencing external config files.

Implemented a sample extension that queried p4 protect information

  to do access control for Perforce repositories. This implementation is

  visible in the documentation for the hook.
Ran unit tests.