Also, explicitly make it throw an error when it's getting an empty glob.
It used to just return the root in that case, but it's unclear if this
would be the right thing, or maybe return everything, or nothing.
subtree is complete with no need to rehack its root. Simplifies a
bunch of code, and makes some utilities redundant.
* Using a single struct for all nodes, with subs=#f to mark files
* Names of struct is generic, no relation to actual files and dirs
* Added more tests
svn: r17527
empty directory is *not* dropped. This finally makes the results more
intuitive, and can now also express queries like only directories,
without their content. (See comments and new tests.)
svn: r17456
