Review Board keeps track of the entire history of your review request.
Every new diff, every field change, every file attachment, every comment.
It's all there, preserved for all time. You can go back and see why a
change was made, how a change evolved, and what issues were raised.
It even shows a nice diff of the text fields, even if they include
Markdown text or images.
Third-party extensions can also provide custom fields, which will be
included in the history, with no effort.
The history is also fully available through the API, allowing for integration
for reporting and analytics.