commands: directly use exchange.pull localrepo.pull is going away. See explanations in 4d52e6eb98ea.

exchange: have `pull` return the pulloperation object We mimic what was done for `push` for similar reason. We are about to drop `localrepo.pull` (for consistency with dropping `localrepo.push` and we better have an API as extensible as `push` is. Find explanations about localrepo.push removal in 4d52e6eb98ea.

revset: introduce an abstractsmartset class This class documents all methods required by a smartset. This makes it easier for people to respect the API and ensure we fail loudly when something does not. It will later also contain common default implementations for multiple methods, making it easier to have smartset classes with minimal work.

revset: add a `__nonzero__` to baseset We are about to add a base class for `baseset` with an abstract `__nonzero__` method. So we need this method to be explicitly defined to avoid issues. The built-in list object apparently does not have a `__nonzero__` and relies on `__len__` for this purpose?

revset: drop isinstance(baseset) in spanset.__sub__ As baseset now has a fast __contains___ operator, this `baseset.set()` dance is no longer needed. No regressions are visible in the benchmark.

revset: drop isinstance(baseset) in spanset.__and__ As baseset now has a fast __contains___ operator, this `baseset.set()` dance is no longer needed. No regressions are visible in the benchmark.

revset: drop isinstance(baseset) from baseset.__and__ As baseset now has a fast __contains___ operator, this `baseset.set()` dance is no longer needed. No regressions are visible in the benchmark.