R1:869f24bf333a

Fix reticle for diff-of-diffs

Summary:
The older logic was incorrect:

- It chose `change.left` for `data.on_right` being true.
- 'O' and 'N' mean 'old' and 'new', not 'left' and 'right'. In diff-of-diffs, both sides are 'N'.

So, select the changeset ID correctly (pick the right side one for on_right), and select the new file prefix correctly (N for new, O for old).

Test Plan: Waved my mouse over some inline comments in a diff-of-diffs, got reasonable-looking reticles.

Reviewers: vrana, btrahan…