Add a "branches" rule for Herald commit rules

Summary:
Fixes T4195. Allows you to write a rule against a commit's branches.

This completes outstanding work on T4195.

Test Plan: Pushed to Git and Mercurial repositories and verified branches were selected correctly by examining transcripts.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7820