To use this, symlinks need to be set up to /etc/nixos, this should be handled by
the git post commit hook.  common/ should be symlinked as a subdirectory of
/etc/nixos, and then the file for the given machine should be symlinked as
/etc/nixos/configuration.nix.  Also, the nixpkgs/config.nix will be symlinked in
user's home directory.

Included is a script that can be used as a git hook to accomplish this.  Set it
up like so:

cd .git/hooks
for i in commit merge checkout rewrite; do
    ln -sf ../../setup-symlinks post-$i
done

It will now be run any time changes are made to the repository.