checkheads: don't warn about unsynced changes that we ill obsolete We won't be able to pull them after this push.

checkheads: check successors for new heads in both missing and common A relevant obsolete marker may have been added -after- we previously exchanged the changeset. We have to search for remote heads that disappear by the sole fact of pushing obsolescence. This case will also happen when remote got the new version from a repository that does not propagate obsolescence markers.

checkheads: attend to phases when computing new heads with obsolete Checkheads was more permissive than expected. When the remote heads are public we don't need to search for successors. None will make a public head disappear.