Ensure a Note's st_diff attribute is properly set

First, for reasons unknown, diff_for_line_code wasn't being called for MergeRequest noteables. So we removed that check.

Second, when it was being called, it didn't do a good enough job of returning a diff -- it could return a Note record that didn't even have a diff itself!

So if a user posted a reply to an outdated diff discussion, the new note's st_diff would be nil, and the frontend wouldn't know how to display it ("It's got a line code but no diff? 💥").

Now the reply gets a diff from its "parent" note (if one's available), and everyone's happy.