Allow searching in repositories

Summary: We may later integrate it in the global search but I want to leave it here too for the case that you want to search just some repository or some part of it.

Test Plan: Browsed repo in SVN and Git, searched in Git.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, chad

Differential Revision: https://secure.phabricator.com/D5738