R1:b1e1b1f9bdf4

Basic support for Mercurial in Diffusion

Summary: Change import script plus almost all the view stuff. Still some rough
edges but this seems to mostly work. Blame is currently unsupported but I think
everything else works properly.

Test Plan:
Imported the hg repository itself. It doesn't immediately seem completely
broken. Here are some screens:

https://secure.phabricator.com/file/view/PHID-FILE-1438b71cc7c4a2eb4569/
https://secure.phabricator.com/file/view/PHID-FILE-3cec4f72f39e7de2d041/
https://secure.