pull: move `fetch` subset into the object Tree discovey use a `fetch` variable to know what is being pulled. We move this information in the `pulloperation` object. This make it possible to extract the changeset pulling logic into its own function.

pull: make pulled subset a propertycache of the pull object The computation of the subset is simple operation using two useful pull information (1) the set of common changeset before the pull (2) the set of pulled changeset. We move this data into the `pulloperation` object since some phase will need them. And we turn the pulled subset computation behind a property case as multiple pull phase will need it.

pull: move phases synchronisation in its own function Now that every necessary information is held in the `pulloperation` object, we can finally extract the phase synchronisation phase to it's own function. This changeset is pure code movement only.

pull: move pulled subset into the object We compute the set of local changeset that were target of the pull. This is then used by phases logic to decide which part of the history should have it phase updated. We move this information into the object to allow extraction of phase synchronisation in its own function. I expect obsolete marker exchange to use it too in the future.

revset: added operations to spanset to duck type baseset Added more operations which are not lazy but only used so far to duck type baseset.

revset: added basic operations to spanset Added methods __add__, __sub__ and __and__ to duck type more methods in baseset

revset: added spanset class to represent revision ranges

revset: added lazyset implementation to extra revset