exchange: don't attempt phase exchange if phase-heads was in bundle The Mercurial core server doesn't yet include phase-heads parts in the bundle, but our Google-internal server wants to do that. Unfortunately, the usual exchange still happens even if phase-heads part is included (including the short-circuited one for old/publishing servers). That means that even if our server (again, the Google-internal one, but also future Mercurial core servers) includes a phase-heads part to indicate that some heads should be drafts, that would still get overwritten by the phase updating that happens after. So let's fix that by marking the phase step done if we receive at least one phase-heads part in the bundle. Differential Revision: https://phab.mercurial-scm.org/D440