Not sure why I chose ProcessPoolExecutor in the first place.

Printing the changed file and new version can be used to commit the changes to git.

Update scripts can now declare features using

	passthru.updateScript = {
	  command = [ ../../update.sh pname ];
	  supportedFeatures = [ "commit" ];
	};

A `commit` feature means that when the update script finishes successfully,
it will print a JSON list like the following:

	[
	  {
	    "attrPath": "volume_key",
	    "oldVersion": "0.3.11",
	    "newVersion": "0.3.12",
	    "files": [
	      "/path/to/nixpkgs/pkgs/development/libraries/volume-key/default.nix"
	    ]
	  }
	]

and data from that will be used when update.nix is run with --argstr commit true
to create commits.

We will create a new git worktree for each thread in the pool and run the update
script there. Then we will commit the change and cherry pick it in the main repo,
releasing the worktree for a next change.

Get rid of some globals, split main into smaller functions, rename some variables, add typehints.

This will make it cleaner and also better respect SIGTERM.

…trPath

Instead of having the updateScript support returning JSON object,
it should be sufficient to specify attrPath in passthru.updateScript.
It is much easier to use.

The former is now considered experimental.

…utes

We can determine all of them when attrPath is present so we might jsut as well do it.

…mmitted

We no longer need it for most use cases so I am making it experimental.

I have something in mind where it might be useful in the future (customizing commit messages)
but for now, it would only confuse people.

…R_PATH

The environment variable will contain the attribute path the script is supposed to update.

`update.nix` extracts `passthru.updateScript` attributes in the main repo
and when they are relative paths (e.g. `./update.sh`), Nix will resolve them
to absolute paths in the main repo.

Update scripts can use $(dirname $0) to get the location of files they
should update but that would point to the main repo.
We want them to modify the appropriate git worktree instead
so we replace the prefix accordingly.

`git rev-parse --show-toplevel` will resolve symlinks but, fortunately,
Nix will do that as well, so the path will match:

#98304 (comment)

- Make some arguments more fitting (the path is actually full, not just relative to prefix…).
- Increase the purity of packages* functions (they now take pkgs from argument, not from scope).
- Add some documentation comments.

maintainers/scripts/update.nix: Add support for auto-commiting changes