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.
If we want to push only one branch, we'll have to specify branch and
remote explicitly. Pushing to origin doesn't work for everyone, since
some of us have a origin remote that can't be pushed to. Using plain
`git push` has the problem that it'll try pushing all checked out
branchs which fails e.g. if some branches (staging, staging-next, …) are
behind their remote counterparts.
The solution is to require everyone to configure a per branch pushRemote
for haskell-updates which will then be used by merge-and-open-pr.sh.
