I'm working on making it so "Update Diff" behaves/looks similar to the

pre-commit section of the "New Review Request" page. This change is a

preperatory step: separating the review request creation from the diff

creation. After this change, RB.PreCommitModel should be usable when a review

request already exists, by setting the reviewRequest attribute.

Verified that pre-commit review request creation still worked properly with

both single and parent diffs.