[PATCH 0 of 6 phases] Secret changeset

[pierre-yves.david at logilab.fr]

This series implement secret changeset logic.

Secret changeset are kept local and never exchanged.


The only way to create a secret changeset for now is to add a --secret switch to
commit. The same patch add a new secret parameter to internal machinery. This
addition have a huge impact on extension but I could find another solution as
clean.

We'll probably want simillar switch on unbundle and import but this series focus
on getting the internal mechanism in place.