Basic filename search support for Diffusion

Summary:
Ref T156. Adds basic filename search support for Diffusion,
currently only for Git repositories.

This is preliminary, and it's up for discussion:
  - is the UI in the right place;
  - what should the search query syntax be (e.g. whether
    to put `*`s in the beginning and end of it);
  - how to best approach it for Mercurial and/or SVN;
  - what's the cleanest result format for `lsquery` (I went
    for the minimum necessary change to `DiffusionBrowseSearchController`).

Test Plan:
Browse to a repository in Diffusion, and use both
`Search File Names` and `Search File Content`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T156

Differential Revision: https://secure.phabricator.com/D8093