sshpeer: allow doublepipe on unbuffered main pipe

The output pipe does not have manually managed read buffer (actually, no read
anything). To also use the doublepipe for outgoing write (useful to consume
remote output when pushing large set of data) we need the doublepipe to work on
standard pipe too.

mercurial/sshpeer.py

@@ -65,7 +65,7 b' class doublepipe(object):'
 
         (This will only wait for data if the setup is supported by `util.poll`)
         """
-        if self._main.hasbuffer:
+        if getattr(self._main, 'hasbuffer', False): # getattr for classic pipe
             return (True, True) # main has data, assume side is worth poking at.
         fds = [self._main.fileno(), self._side.fileno()]
         try: