Provide Git and Mercurial commit IDs when posting diffs.

Review Board 2.0 supports associating commit IDs with review requests,
but it must be provided by the client.

RBTools now provides these commit IDs, when returned by the diff call.
These are only provided by Git and Mercurial right now, where they're
more useful and in higher demand.

The commit ID won't be posted if running on older versions of
Review Board.