Add "DiffusionRawDiffQuery"

Summary:

• This is only slightly useful for updating Differential, since DiffQuery (vs RawDiffQuery) already gets you most of what you need. The only thing is that DiffQuery returns the diff for one path only right now(and the SVN version is very "special"). Should be easy to fix in the Git/HG cases at least, though (or maybe just use RawDiffQuery to avoid the SVN mess).
• Added a "download raw diff" link.

Test Plan: Viewed Diffusion and raw commits for SVN, Mercurial and Git repositories.

Reviewers: vrana, btrahan, jungejason

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2350