push: move `newbranch` argument into the push object One more step toward a more modular push function.

push: move `revs` argument into the push object One more step toward a more modular push function.

push: move `force` argument into the push object One more step toward a more modular push function.

push: move `remote` argument in the push object One more step toward a more modular push function.

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.