push: ease access to current ui object The `pushoperation.repo.ui` attribute is very commonly used. we offer a faster way to access it directly through the push operation object.

push: introduce a pushoperation object This object will hold all data and state gathered through the push. This will allow us to split the long function into multiple small one. Smaller function will be easier to maintains and wrap. The idea is to blindly store all information related to the push in this object so that each step and extension can use them if necessary. We start by putting the `repo` variable in the object. More migration in other changeset.

exchange: extract push function from localrepo The localrepo class if far too big. Push and pull logic will be extracted and reworked to better fit with the fact they now exchange more than plain changeset bundle. This changeset extract the push code. later changeset will slowly slice this over 200 hundred lines and 8 indentation level function into smaller saner brick. The localrepo.push method is kept for now to limit impact on user code. But it will be ultimately removed, now that the public supposed API is hold by peer.