Distinguish between ported-forward and ported-backward comments master
Summary: Ref T7447. This might be overkill, but I want to over-explain things until we have more confidence that this is rarely confusing. NOTE: I'm playing it a bit fast and loose with `setIsGhost()` (passing a dictionary) because making API changes requires changing the interface and Diffusion, which is a pain. I'll clean this up at the end once the interface is more final. This is well-contained for now. Test Plan: - Viewed "base vs 2" in a diff with 3 diffs, saw some "older comments" and some "newer comments". - Hovered the tags for an explanation of comment spookiness. {F377703} Reviewers: btrahan Reviewed By: btrahan Subscribers: epriestley Maniphest Tasks: T7447 Differential Revision: https://secure.phabricator.com/D12490