Fix#24623
Major changes:
1. Redirect `/owner/repo/blob/*` requests to `/owner/repo/src/commit/*`
(like GitHub)
2. Add a "view file diff" link (see screenshot below)
3. Refactor "AssertHTMLElement" to generic, now we can accurately assert
existence or number.
4. Add more tests
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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <dev.lh@web.de>