Check that all the Haskell resources are in the git repository, and warn otherwise

When cabal is run from the root of a git repository, I would love for it to check that all the Haskell resources are in the git repository, and warn otherwise. It's very easy to write a new module, test it, etc., and then forget to actually add it before pushing a broken commit. Note: a warning ideally should *not* be generated if the file is staged to be added to the repository.