Currently, every package set consists of three commits, generated by
update-hackage.sh, update-stackage.sh and
regenerate-hackage-packages.sh, respectively. This is suboptimal, as it
necessarly causes intermediate states of Nixpkgs where the generated
hackage-packages.nix and all-cabal-hasehs and/or the hackage2nix
configuration files are out of sync. Ideally, running
regenerate-hackage-packages.sh is a no-op for every Nixpkgs revision.
This is achieved by adding a wrapper script, update-package-set.sh,
which runs the individual moving parts and commits the result.
