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

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

* 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.

Inline diff is going to be generated client side now. #3945

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.