make up-item smart

[aki2o]

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.

(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.

[aki2o]

BTW, The following message was shown on the web page of Github when I send this pull request.

Error checking merge status
Don't worry, you can still submit the pull request

Does this pull request have any problem?

[m2ym]

Commented on the commit.