stablerange: fix slicing of arbitrary ranges Pull bundle trigger slicing from range with arbitrary initial skip. We have to adjust the current slicing to take this into account.

pullbundle: attempt to save stablerange cache after each computation Since we do not serve the full repository, we use more stablerange starting from a random point. These are probably not loaded yet and should be cached. This is best effort caching, if we can't save them we won't.

pullbundle: add basic extension metadata

pullbundle: add a config option for the cache directory

pullbundle: add an output about the time spent slicing the set This will be useful to monitor overall performance.

pullbundle: add extension documentation

pullbundle: add an extra test showing how cache are reusable We issue a single final pull, most of its item are reusable.