Fix bug for nested redoPath calls.

I'm not sure if I quite understand how redo handles subdirectories.
Should I pass in fullpaths, into the scripts? Should I recurse back from
a nested directory into the cwd? How do I handle dependency hashes for
nested targets?

The file-based implementation is indeed not quite what I want, but it
was easy enough to implement, to the point it doesn't really matter, as
of right now.