discovery: add new set-based discovery Adds a new discovery method based on repeatedly sampling the still undecided subset of the local node graph to determine the set of nodes common to both the client and the server. For small differences between client and server, it uses about the same or slightly fewer roundtrips than the old tree-based discovery. For larger differences, it typically reduces the number of roundtrips drastically (from 150 to 4, for instance). The old discovery code now lives in treediscovery.py, the new code is in setdiscovery.py. Still missing is a hook for extensions to contribute nodes to the initial sample. For instance, Augie's remotebranches could contribute the last known state of the server's heads. Credits for the actual sampler and computing common heads instead of bases go to Benoit Boissinot.