IIUC, you're looking for true diffs between revisions with different bases. If so, Reviewable deals with that automatically, running 4 separate diffs in the background and integrating the results so that base-only diffs are hidden by default.

As for diffing commits, you can switch a review into "review each commit" mode so that it will create one revision per commit. You can make this the default for a repo but it's not in the demo as it's not the most common way to work. The demo does really need updating, though!