setdiscovery: use iterbatch interface instead of batch It's a little more concise, and gives us some simple test coverage.

peer: add an iterbatcher interface This is very much like ordinary batch(), but it will let me add a mode for batch where we have pathologically large requests which are then handled streamily. This will be a significant improvement for things like remotefilelog, which may want to request thousands of entities at once.

wireproto: document quirk of _callstream between http and ssh This tripped me up when trying to use it, so it feels like we should document this to avoid future pain.

peer: raise NotImplementedError for abstract submit() method Nothing should ever call this submit method directly as it should be overridden by concrete batcher implementations.

fsmonitor: new experimental extension Extension to plug into a Watchman daemon, speeding up hg status calls by relying on OS events to tell us what files have changed. Originally developed at https://bitbucket.org/facebook/hgwatchman

fsmonitor: dependencies for new experimental extension In preparation for the filesystem monitor extension, include the pywatchman library. The fbmonitor extension relies on this library to communicate with the Watchman service. The library is BSD licensed and is taken from https://github.com/facebook/watchman/tree/master/python. This package has not been updated to mercurial code standards.

setup: show how to set the module policy for imports This is not technically needed, since mercurial.__version__ does not exist as a native module, but, without this style wrappings, if something else had a native flavor, the module loader would get upset. In principle, the `env` object is trying to set HGMODULEPOLICY for children, so, conceptually we should set it for this in-process child.