Add diffing.

Sometimes it might be helpful to show the diff between commits rather than just the state at a given commit.

Admittedly this is likely a much harder task, but might be useful. :)

Start by reviewing the React component's current behavior for displaying the state at a given commit and how commit references are handled. Define the expected diff inputs and presentation before implementation; done means the component can show differences between commits rather than only one commit's state.

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