Add branch as an option to recentcommitsbypath API

Summary: We need to be able to request history for more than the master branch (the default). This adds branch as an option to the API.

Test Plan: Test by sending recentcommitsbypath a non-master branch along with callsign.

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5967