Ensure a Note's st_diff attribute is properly setrs-note-st_diff

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So if a user posted a reply to an outdated diff discussion, the new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? :boom:").

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

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