vocabs.hierarchy: faster all-disk-vocabs-recursive.

vocabs.hierarchy: faster all-disk-vocabs-recursive.

The old technique caused a high amount of redundant ``exists?`` checks,
even though we are traversing the directory tree.  That happens to be a
little slow on Windows, for some pathological reason, the first time
it's run. This should make it better while we also investigate why
``windows_stat`` is slower in that case.