So we have raw_diffs too

This object will manage Gitlab::Git::Compare instances

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 reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Signed-off-by: NRémy Coutable <remy@rymai.me>

Also fix one use of `gsub` that would be faster as `delete`.

Use `tr` instead of `gsub` when you are replacing the same number of
characters. Use `delete` instead of `gsub` when you are deleting
characters.

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.