tmux opens splits in same directory; fix for tmux 1.9+.

tmux 1.9 CHANGELOG[1] says:

* 'default-path' has been removed.  The new-window command accepts '-c' to
  cater for this.  The previous value of "." can be replaced with: 'neww -c
  $PWD', the previous value of '' which meant current path of the pane can
  be specified as:  'neww -c "#{pane_current_path}"'

[1]: http://sourceforge.net/p/tmux/tmux-code/ci/master/tree/CHANGES