util: handle fileno() on Python 3 throwing io.UnsupportedOperation

Fortunately, the exception exists on Python 2 so we don't have to do
something weirder than this.

Differential Revision: https://phab.mercurial-scm.org/D2450

mercurial/util.py

@@ -26,6 +26,7 b' import errno'
 import gc
 import hashlib
 import imp
+import io
 import itertools
 import mmap
 import os
@@ -1178,7 +1179,10 b' def _sethgexecutable(path):'
 
 def _isstdout(f):
     fileno = getattr(f, 'fileno', None)
+    try:
     return fileno and fileno() == sys.__stdout__.fileno()
+    except io.UnsupportedOperation:
+        return False # fileno() raised UnsupportedOperation
 
 def shellenviron(environ=None):
     """return environ with optional override, useful for shelling out"""