Expose repository ref rules via "diffusion.repository.search"

Summary:
Depends on D20425. Ref T13277. See PHI1067. There's currently no way to retrieve branch/ref rules over the API, which makes some management operations against a large number of repositories difficult.

Expose these rules to the API.

Test Plan: Called diffusion.repository.search, got rules in the result set.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13277

Differential Revision: https://secure.phabricator.com/D20426