Mocking isolated directories

[smtlaissezfaire]

It would be great if we could just isolate / mock one directory, then use everything else from the regular fs. This might also be a good solution for #462

An issue I ran into: I used include FakeFS::SpecHelpers in a spec, but then the calling code tried to load a model which wasn't previously loaded. The model file (in app/models/x.rb) wasn't in the fake file system.

This almost seems to be the opposite of FakeFS::FileSystem.clone.

[grosser]

would be lots of rework afaik ... touching almost every file, so probably not going to happen
I'd usually recommend to just use Dir.mktmpdir for hard to test cases