progress: make sure stderr has isatty before calling (issue2191)

hgext/progress.py

@@ -51,7 +51,8 b' def spacejoin(*args):'
     return ' '.join(s for s in args if s)
 
 def shouldprint(ui):
-    return sys.stderr.isatty() or ui.configbool('progress', 'assume-tty')
+    return (getattr(sys.stderr, 'isatty', None) and
+            (sys.stderr.isatty() or ui.configbool('progress', 'assume-tty')))
 
 class progbar(object):
     def __init__(self, ui):