Fix merge request comments on files with multiple commits

Created by: dsander

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• Do not rely just on outdated? of the first note when displaying comments of a MR in the discussion tab, but partition the active/outdated notes and display them all
• In the inline changes tab just select the active notes, so an outdated note does not prevent an active one from being rendered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