refs: allow overwrite of empty folders

refs: allow overwrite of empty folders

We noticed that once a ref folder is created, there is no existing
command that can remove it. For example, once "foo/bar" is created,
even if the user deletes foo or all the refs under foo, the folder
will persist.

Now when the user attempts to create a ref "foo" either through commit
or refs --create, if a folder "foo" exists but is empty of refs, the
folder is removed and the new ref "foo" is created.

New unit tests in tests-ref.sh verify this functionality.

Closes: #354
Approved by: cgwalters