Add a SublimeText-style repository typeahead

Summary:
Allows you to quickly search for files within a repository. Roughly:

• We build a big tree of everything and ship it to the client.
• The client implements a bunch of Sublime-ish magic to find paths.

Test Plan: {F154007}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley, zeeg

Differential Revision: https://secure.phabricator.com/D9087