ui: flush before prompting for input with readline

I was using `hg absorb` with Python 3 and noticed that the
prompt was appearing without any output about what would be
done. After I answered the prompt, the output was printed
to stdout.

This appears to be a buffering difference between Python 2
and Python 3.

To work around it, this commit adds an explicit flush() before
calling the raw input function when readline is used.

mercurial/ui.py

@@ -1564,6 +1564,7 b' class ui(object):'
         # - http://bugs.python.org/issue12833
         with self.timeblockedsection(b'stdio'):
             if usereadline:
+                self.flush()
                 prompt = encoding.strfromlocal(prompt)
                 line = encoding.strtolocal(pycompat.rawinput(prompt))
                 # When stdin is in binary mode on Windows, it can cause