R1:958b00c01016

Diffusion: encapsulate request in DiffusionRequest

Summary:
Put an indirection layer between controllers and URI management,
adding branches to git repositories.

Test Plan:
Looked at browse, history browse, file browse views, bad branches,
bad commits

Reviewed By: jwilson
Reviewers: aran, jwilson
CC: jwilson, epriestley
Differential Revision: 65