This adds a method to track errors that can be recovered from in
sentry.

It is useful when debugging performance issues, or exceptions that are
hard to reproduce.

Rugged sometimes chops a context line in between bytes, resulting in the context
line having an invalid encoding: https://github.com/libgit2/rugged/issues/716

When that happens, we will try to detect the encoding for the diff, and
sometimes we'll get it wrong. If that difference in encoding results in a
difference in string lengths between the diff and the underlying blobs, we'd
fail to highlight any inline diffs, and return a 500 status to the user.

As we're using the underlying blobs, the encoding is 'correct' anyway, so if the
string range is invalid, we can just discard the inline diff highlighting. We
still report to Sentry to ensure that we can investigate further in future.

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.