win32: assign winstdout to sys.__stdout__ as well (issue2888)

On Windows sys.stdout was being replaced with winstdout, which caused
util.system() to redirect its output (due to 388eabdda556). That causes
interactive tools (such as vim) to stop working.

mercurial/windows.py

@@ -62,7 +62,7 @@ class winstdout(object):
             self.close()
             raise IOError(errno.EPIPE, 'Broken pipe')
 
-sys.stdout = winstdout(sys.stdout)
+sys.__stdout__ = sys.stdout = winstdout(sys.stdout)
 
 def _is_win_9x():
     '''return true if run on windows 95, 98 or me.'''