push: feed pulloperation object to _pullobsolete function We now have all necessary information in the `pulloperation` object itself.

pull: move transaction logic into the pull object Most local change that occurs during a pull are doing within a `transaction`. Currently this mean (1) adding new changeset (2) adding obsolescence markers. We want the two operations to be done in the same transaction. However we do not want to create a transaction if nothing is added to the repo. Creating an empty transaction would drop the previous transaction data and confuse tool and people who are still using rollback. So the current pull code has some logic to create and handle this transaction on demand. We are moving this logic in to the `pulloperation` object itself to simplify this lazy creation logic through all different par of the push. Note that, in the future, other part of pull (phases, bookmark) will probably want to be part of the transaction too.

pull: move obsolescence marker exchange in the exchange module The obsolescence marker exchange code was already extracted during a previous cycle. We are moving the extracted functio in this module. This function will read and write data in the `pulloperation` object and I prefer to have all core function collaborating through this object in the same place. This changeset is pure code movement only. Code change for direct consumption of the `pulloperation` object will come later.

pull: move `force` argument into pull object One more step toward a more modular pulh function.