Improve repo commit view (#33877)

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

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <dev.lh@web.de>

tests/integration/oauth_test.go

@@ -77,7 +77,7 @@ func TestAuthorizeShow(t *testing.T) {
 	resp := ctx.MakeRequest(t, req, http.StatusOK)
 
 	htmlDoc := NewHTMLParser(t, resp.Body)
-	htmlDoc.AssertElement(t, "#authorize-app", true)
+	AssertHTMLElement(t, htmlDoc, "#authorize-app", true)
 	htmlDoc.GetCSRF()
 }