Git pre-commit/push hook

[infinisil]

We should have this in Nixpkgs for https://github.com/NixOS/nixpkgs/pull/326407. Here are some notes for that:

• For every push/commit, it should verify that changed and new files are formatted
  • For every commit is more useful because like that no extra commits will be necessary just to format
  • But for every push would also have advantages, see https://github.com/NixOS/nixpkgs/pull/322537#issuecomment-2214466166
  • Ideally this could also be together with a rewrite/rebase mode, kind of like https://github.com/grahamc/git-rebase-format/blob/master/git-rebase-format.sh (ref #159), which would allow users to rebase their PRs, maintaining the same commits, but formatting each of them
    • Probably something with git filter-branch would be much simpler than that script though
• It should use the nixfmt version pinned in the shell.nix
• It should install itself into .git/hooks automatically via shellHook (this is also how https://github.com/cachix/git-hooks.nix works)
• Not a requirement, but ideally it would share as much with the GitHub Action workflow as possible. It's notably fairly easy to get another Git checkout of the base branch.

Ping @emilazy