[PATCH 0 of 4 phases] refactoring findcommonoutgoing and use it to have a clean implementation of phase.

[Pierre-Yves David]

The first patchs of this  series are a refactoring for findcommonoutgoing.

The final goal is to simply the whole business to allow an easy and clean
exclusion for secret changeset from discovery.

The fact that discovery can eventually discover common secret changeset are not
handled by this series. Neither the fact than common contains common who may
not be ancestor of target heads. Those both issue are to be taken care later.