util: forward __bool__()/__nonzero__() on fileobjectproxy

In trying to debug the Windows process hang in D2720, I changed the stderr pipe
to the peer to be os.devnull instead.  That caused sshpeer._cleanuppipes()[1] to
explode, complaining NoneType has no __iter__ attribute, even though the
previous line checked for None.

[1] https://www.mercurial-scm.org/repo/hg/file/b434965f984e/mercurial/sshpeer.py#l133

mercurial/util.py

@@ -543,6 +543,11 b' class fileobjectproxy(object):'
 
         return getattr(object.__getattribute__(self, r'_orig'), name)
 
+    def __nonzero__(self):
+        return bool(object.__getattribute__(self, r'_orig'))
+
+    __bool__ = __nonzero__
+
     def __delattr__(self, name):
         return delattr(object.__getattribute__(self, r'_orig'), name)