Set pane_current_path

ChangeLog (1.8->1.9) 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}"'
```