make up-item smart

I suppose it's better to move the parent directory if the root directory of the direx buffer has a parent directory.

I tested that the changed behavior of this pull request is up to my expectation.
But, I'm not sure whether the following code in `direx:up-item` is OK or not.

```lisp
(erase-buffer)
(direx:add-root-into-buffer it (current-buffer))
(rename-buffer (direx:tree-name it) t))
```

Does this code give any bad effect to the other function of direx?

Best regards.