In this specific case using file_path as a cache key is not enough,
because there are two entries with the same path.
Closes #21610.

Instead calling diff_collection.count use diff_collection.size which is cache on the diff_collection

Introducing the concept of SafeDiffs which relates 
diffs with UI highlighting.

When rendering a list of diff files, skip those where the diff is over
10 KB and provide an endpoint to render individually instead.

Extract parts of LegacyDiffNote into DiffOnNote concern and move part of responsibility to other classes

This builds on the changes introduced in
https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 and results
in merge requests with large diffs (e.g. due to them containing minified
CSS) loading much faster.

* Use commit objects instead of IDs when generating diffs
* Use proper references when generating MR's source and target
* Update broken specs

Now we apply syntax highlighting to the whole old and new files.
This basically help us to highlight adequately multiline content.

* It is more performant given now we process all the diff file instead
  of processing line by line.
* Multiline comments are highlighted correctly.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>