ui: do not try readline support if fin/fout aren't standard streams

It's unlikely for a non-stdio stream to be a tty. Minimizing readline support
makes it much simpler to work around the unicode input() function of Python 3.

This also works on chg which duplicates client's tty to stdio fds.

mercurial/ui.py

@@ -1264,7 +1264,9 b' class ui(object):'
         return i
 
     def _readline(self):
-        if self._isatty(self.fin):
+        usereadline = (self._isatty(self.fin) and self._isatty(self.fout)
+                       and util.isstdin(self.fin) and util.isstdout(self.fout))
+        if usereadline:
             try:
                 # magically add command line editing support, where
                 # available
@@ -1273,7 +1275,7 b' class ui(object):'
                 readline.read_history_file
                 # windows sometimes raises something other than ImportError
             except Exception:
-                pass
+                usereadline = False
 
         # prompt ' ' must exist; otherwise readline may delete entire line
         # - http://bugs.python.org/issue12833