R1:2d27324bef95

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…