D4120: shortest: use nodetree for finding shortest node within revset

[Yuya Nishihara]

test-revisions.t fails probably because null. I've queued up to the previous
patch.

>  static int nt_init_py(nodetree *self, PyObject *args)
>  {
> -	PyObject *index;
> +  indexObject *index;

Dropped this change as it's unrelated to this patch, and the index variable
is casted to indexObject* anyway.

>  	unsigned capacity;
> -	if (!PyArg_ParseTuple(args, "O!I", &index, &capacity))
> +	if (!PyArg_ParseTuple(args, "O!I", &indexType, &index, &capacity))
>  		return -1;

Moved this to D4118.

>  	return nt_init(self, (indexObject*)index, capacity);