Python 3.5 adds type-checking to difflib, which causes gundo to fail.
This fixes the issue by converting the arguments to strings.
See https://bugs.python.org/issue17445 for details
Instead of displaying hardcoded "j/k" for motion between undo states,
real keymaps are fetched from `g:gundo_map_move_older` and
`g:undo_map_move_never` variables.
