mv: cleanup empty WORKING_DIRECTORY

mv: cleanup empty WORKING_DIRECTORY

Originally, moving from-in-to-out may leave an empty <source>
directory on-disk (this kind of directory is marked as
WORKING_DIRECTORY).

Cleanup such directories if they are empty (don't have any entries
under them).

Modify two tests that take <source> as WORKING_DIRECTORY to test
this behavior.

Suggested-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>