Install rust forge triagebot for range-diff comments

[roberth]

Is your feature request related to a problem?

We rebase quite often, but that can make the increment hard to review, because unfortunately GitHub only shows the naive diff. It should show a range-diff. Triagebot can add that info for us instead.

Here it is in action.
The range diff shows changes only in the diff context, showing that there were no actual code changes in my force push.

Proposed solution

1. Install https://forge.rust-lang.org/triagebot/index.html
2. Enable automatic range-diff comments: https://forge.rust-lang.org/triagebot/range-diff.html

Alternative solutions

Ask GitHub for a native feature? Better outcome eventually but unpredictable timeline.

Additional context

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