Strip subtrees which are now completely empty (issue #1)

Git tools apparently don't like empty trees much (C-Git CLI certainly
makes it difficult to create empty trees, though it doesn't seem to mind
them if it encounters them?).

Also, if we're using multiple-blob-id removal to nuke directories (as
in issue #12) then leaving the empty husks of directories around will
just be ugly.

https://github.com/rtyley/bfg-repo-cleaner/issues/12#issuecomment-14668029