Support relative references/revisions using the git-type fetchers

I want to be able to specify a relative git rev, such as `<hash>~1`:
```
nix run "github:nixos/nix?rev=dfc876531f269950a4e183a4f77a813c421d7d64~1" -- --extra-experimental-features nix-command hash file --type sha256 --base32 <(printf "Why, John Ericson, why?")
```
to demonstrate how things change from child to parent.
