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

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.

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