phabricator: add commenting to phabsend for new/updated Diffs Especially useful when sending updates to existing Revisions so one can specify the sort of changes e.g. "Address review comments" or "Rebase to tip" If the diff content hasn't changed then it only needs a metadata update which doesn't show in the Phabricator updates UI, so don't add a comment that will. Differential Revision: https://phab.mercurial-scm.org/D6496