Right now, it shows diff between first `arc diff` and last `arc diff`. Comments from reviewers are lost in between. It would be nice if Differential pre-selected last diff with comments and last `arc diff` call. Either that, or highlight `arc diff` calls with review comments.
For example: {D9140}. Say @epriestley (author) or @btrahan (reviewer) opened the revision after @epriestley had sent the last `arc diff`. They would be most likely interested in viewing https://secure.phabricator.com/D9140?vs=21708&id=21727 rather than https://secure.phabricator.com/D9140. For everybody else, full diff should be fine.