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

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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