As a reviewer I want to see only changes added to the review.
Suppose we have two users A and B working on the same file. They are committing their changes in order: A1 -> A2 -> B -> A3. Then user A creates a review with all his changes.
This is what collaborator Diff Viewer produces:
What we expect to see is all user A lines marked green, and line from user B left white.
This is a common situation that multiple users work on the same file and we review the same piece of code twice, or even review code that no longer exists.