sshpeer: move handshake outside of sshpeer With the handshake now performed before a peer class is instantiated, we can now instantiate a different peer class depending on the results of the handshake. Our test extension had to change to cope with the new API. Because we now issue the command via raw I/O calls and don't call _callstream(), we no longer have to register the fake command. (_callstream() uses the command registration to see what args to send). Differential Revision: https://phab.mercurial-scm.org/D2034