A way to diff that only recurses if outputs are different

I have two derivations that are different, `/nix/store/f6a70iijzdq7xzkyycqcggqdv2s77nla-HsOpenSSL-0.11.4.11.drv` and `/nix/store/7kmc4asp07ffp9ppihmvl0xjk6qcq2gq-HsOpenSSL-0.11.4.11.drv`. However, this doesn't actually matter, because the outputs of both of these derivations are the same thing. The differences in derivation are due to how tars are fetched (different mirrors), but the result is the same due to the fixed output.

Thus while these are different derivations, they don't affect substitution. It'd be nice if `nix-diff` didn't consider such derivations further (just check that the outputs are the same).