Format and behavior of RevisionDiff is still undecided; without it we are collectively unable to implement get_diff_with_parent() in any of the VCS' (or for that matter, revision.diff(a,b)). I suspect that since this is a non-trivial problem (ie. hashing out a common method to represent diffs in a non-specific way), this will not be a feature that will make it to the code freeze by the end of term.

For Git, the stop-gap solution will be to return a raw GitPython Diff object, which is clearly not ideal. But for now it is better than nothing, and given documentation, can be worked around to still produce usable information.