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

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 406b6d7bdcb9). That causes
interactive tools (such as vim) to stop working.

--- a/mercurial/windows.py	Sat Jul 09 19:06:59 2011 +0300
+++ b/mercurial/windows.py	Sat Jul 16 15:24:28 2011 +0300
@@ -62,7 +62,7 @@
             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.'''