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 default policies.

Test Plan:
  - Created a diff with no repositoryPHID.
  - Created a diff with a repositoryPHID.
    - Verified it carried over when the diff was used to create a revision.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: ptarjan, jamesgpearce, epriestley

Differential Revision: https://secure.phabricator.com/D9023