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.

push: move outgoing check logic in its own function Now that every necessary information is held in the `pushoperation` object, we can extract the part responsible of aborting the push to it's own function. This changeset is mostly pure code movement. the exception is the fact this function returns a value to decide if changeset bundle should be pushed.