SCMClient.diff() can now return a review_request_extra_data

dictionary containing state to store on the review request's

extra_data.
If posting a brand-new review request, this will be stored directly on

the review request. If updating an existing one, this will be stored on

the draft.
This can be useful for storing metadata that should show up in fields on

a review request, or for providing metadata that can be used for

enhanced matching of review requests when using rbt post -u (a

feature coming for RBTools 3.1 in an upcoming change).
The data will be sent to Review Board through a JSON Merge Patch.

Unit tests pass for all supported versions of Python.
Modified the Git diff output to include some nested metadata and posted

locally. Verified the results in the posted review request, and then in the

draft.
Tested an earlier version of this with the SOS work, verifying that the data

got into the review request and was displayed in the custom fields.