push: extract new common set computation from phase synchronisation Now that every necessary information is held in the `pushoperation` object, we can extract the new common set computation to it's own function. This changeset is pure code movement only.

push: move `commonheads` into the push object The phase synchronisation start by computing the new set of common head between local and remote and then do the phase synchronisation on this set. This new common set logic will eventually be used by the obsolescence markers exchange. So we are going to split the long phase synchronisation in two.

push: move discovery in its own function Now that every necessary information is held in the `pushoperation` object, we can extract the discovery logic to it's own function. This changeset is pure code movement only.