Fix merge request comments on files with multiple commits

Having a merge request with a comments on a line which is then changed
in a later commit prevented new comments from properly showing up in the
merge request show page.

 * `Note#set_diff` do not use stored the diff when creating a new note
   in merge requests (we can not be sure the diff did not changed since
   the last comment on that line)
 * Do not rely just on `outdated?` of the first note when displaying
   comments of a MR in the discussion tab, but partition the
   active/outdated notes and display them all
 * In the inline changes tab just select the active notes, so an
   outdated note does not prevent an active one from being rendered
 * Just show active comments in the side-by-side changes tab