obsolete: do no complain when push create a new head but obsolete an old one.
Do not read the code or your eyes will burn.
The (+1 heads) message still appear (see mercurial issue 3394).
obsolete: refuse to push unstable changeset without -f
We do not alter phase of suspended and unstable changeset anymore. But push
refuse to push them without force.
The extinct part of the history stay secret for simplicity shake.