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).