rc.go: include path in fileHash

This makes two identical files at different locations
require separate authorization. Fixes #83

I had to remove the test/allow files, and _always_ run `direnv allow` during tests, since the allow behaviour now depends on what path your checkout lives at. So hopefully that's OK.

Also, there's an error message in the tests:

    direnv: error .envrc is blocked because unknown. Run `direnv allow` to approve its content.

But that doesn't actually seem to fail anything, and I can't figure out what .envrc it's referring to, so perhaps you can help with that @zimbatm?