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.

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

* This is because is not a good idea to modify the original lines
* Also I run into this issue https://gitlab.com/gitlab-org/gitlab_git/issues/14
  which is returning Diff Lines with the same @new_pos value.