R1:a0af5b664377

Make DifferentialChangesetParser explicitly map display to storage for comments

Summary:
This is a followup to D228. Basically, we use "changeset" (and, implicitly,
changesetID) for way too much stuff now.

One thing it can't possibly capture is the complete, arbitrary mapping between
the left and right sides of the displayed diff and the places we want to store
the left and right side comments. This causes a bunch of bugs; basically adding
inline comments is completely broken in diff-of-diff views prior…