[PATCH 00 of 12] Phase concept, first changeset

[pierre-yves.david at logilab.fr]

Here is the first set of changeset needed to have a phase concept in mercurial.

This include:

    * phase boundary move when changeset are exchanged
    * rebase and mq refuse to work on immutable changeset

This series broke several test as support for strip and rollback have been
excluded of this first series. Most rebase test broke as initial tree is pulled
from a bundle and therefor immutable.