R1:997c8591b27f

Add 'repositoryPHID' to 'differential.createrawdiff'

Summary:
See <https://github.com/facebook/phabricator/issues/596>

Broadly, Facebook would like to bring Pull Requests from GitHub into Phabricator.

In the long term we can do this properly via Doorkeeper/Nuance, but that's probably a ways off. This seems like a reasonable low-budget compromise for now.

I'm a little hesitant to add a ton of parameters to this call, but `repositoryPHID` seems pretty reasonable, and is notable because it also controls…