R1:7c413fc580f4

Rough version of DiffusionHomeController

Summary: My strategy here is to synthesize Diffusion + Confusion (jwilson's
git support for Diffusion) + Repository/Commit stuff into Phabricator and
kill all birds with one stone. This probably involves the least total work
since we have to do this port step anyway and a lot of the Phabricator
primitives are in better shape than the trunk primitives.

Test Plan: Looked at it in my sandbox, which has some parsed commits.

Reviewers: jwilson, aran, jungejason

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