Thresh already notices when the same comparison has new content and appends another review round. It also has most of the machinery for exact and abridged interdiffs, but nothing actually constructs an interdiff subject yet.
When the contributor posts round two, I want to review just what changed since round one and also be able to read the complete second round. This needs interaction design: should the interdiff be offered automatically, selected by default, or simply called out as ready? Whichever direction we take, switching between the interdiff and the complete round should be obvious and comments should keep showing up near where they were left.
Keep this to consecutive rounds for now. Opening the interdiff must not imply that either round has been read or accepted, and failures should leave the complete exact round viewable.