exchange: extract pull function from localrepo The localrepo class if far too big. Push and pull logic are extracted and reworked to better fit with the fact we exchange more than bundle now. This changeset extract the pulh code. later changeset will slowly slice it into smaller brick. The localrepo.pull method is kept for now to limit impact on user code. But it will be ultimately removed, now that the public API is hold by peer.

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.

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.

push: move `incoming` into the push object The fact that there is some unknown changes on remote one of the result of discovery. It is then used by some push validation logic. We move it in the object to be able to extract the said logic.

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

push: move `remoteheads` into the push object The heads of the remote repository are used to detect race when pushing changeset. We now store this information in `pushoperation` object to allow extraction of the changeset pushing part.