Add route with constraint and rapid diffs view for Commit

What does this MR do and why?

Added a new controller action and view that gets rendered when visiting (/NAMESPACE/PROJECT/-/commit/IID?rapid_diffs=true).

Its available when the rapid_diffs feature flag is enabled and the param rapid_diffs=true is supplied.

See #451368 (comment 1826496267) for more context and !147686 (merged) for a previous example of the same idea.

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

• Enable rapid_diffs feature flag
• Visit /NAMESPACE/PROJECT/-/commit/IID?rapid_diffs=true.
• Note the text Rapid Diffs on above the diff section.

Related to #451785 (closed)