combine_cmds: shlex.split() doesn't get along well with unicode

Just uncovered this while doing some recreational opt dict refactoring:

``` python
>>> shlex.split(u'py3k')
['p\x00y\x003\x00k\x00']
>>> shlex.split('py3k')
['py3k']
```

This is 'fixed' by changing `combine_cmds` to use `shlex.split(str(cmd))`.

This happens if you use **Python 2.7 JSON** but not `simplejson`!
