sshpeer: make pipe polling code more explicit

"hasbuffer" is a property on our special bufferedinputpipe class.
When reading this code, I thought it might have had something
special to do properties on built-in types. But "hasbuffer" doesn't
appear in the CPython code base for either 2.7 or 3.7, so the
answer is no.

Let's make the code more explicit about the fact that it deals with
our special bufferedinputpipe type.

Differential Revision: https://phab.mercurial-scm.org/D2382

mercurial/sshpeer.py

@@ -65,8 +65,11 b' class doublepipe(object):'
 
         (This will only wait for data if the setup is supported by `util.poll`)
         """
-        if getattr(self._main, 'hasbuffer', False): # getattr for classic pipe
-            return (True, True) # main has data, assume side is worth poking at.
+        if (isinstance(self._main, util.bufferedinputpipe) and
+            self._main.hasbuffer):
+            # Main has data. Assume side is worth poking at.
+            return True, True
+
         fds = [self._main.fileno(), self._side.fileno()]
         try:
             act = util.poll(fds)