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