[PATCH 0 of 6 phases] Small initial series for the phases concept

[Pierre-Yves David]

Here the initial bits of phases code. This have been rewritten for use roots
instead of heads. This first series does not to much.

The next series will be:

 * phases: add a moveboundary function to move phases boundaries 
 * phases: set new commit in 1-phase
 * phases: add rollback support 
 * phases: gracefully handle strip  and unknown root

The one after should be:

 * phases: set content of added changegroup in 0-phase
 * phases: set changeset pushed as 0-phase in local repo on push
 * phases: set common changeset to public on pull 

The next might be:

    * phases: add a mutable boolean property on context
    * phases: prevent rebase to rebase immutable changeset
    * phases: phases: prevent mq to qimport immutable change

The one after will be related to phases.publish option and pushkey protocol

-- 
Pierre-Yves