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So if a user posted a reply to an outdated diff discussion, the new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.