Work around extra space added by some readline versions.

Work around extra space added by some readline versions.

Apparently some versions of readline (6.0, for me) in some versions of
Python (Ubuntu's python2.6.4-0ubuntu1, for me) have an irritating bug
where they add an extra space to the end of all completions.  This is
particularly annoying for directory completions, as you can't
tab-complete your way into the contents of the directory.  See
http://bugs.python.org/issue5833

This patch, borrowed mostly from Trac, goes in and twiddles the
appropriate variable inside the readline library to make it stop doing
that.  See http://trac.edgewall.org/ticket/8711 for the discussion.

Signed-off-by: Peter McCurdy <petermccurdy@alumni.uwaterloo.ca>