Allow pulling changes from the root repository when using mrgit

[pomek]

📝 Provide a description of the improvement

Right now, many of us execute the following commands to update an environment:

git pull
mrgit sync

Wouldn't it be useful to execute git pull in the root repository (where the mrgit.json file exists)?

By default, this behavior would be disabled. To enable the extra feature, we should extend the configuration. Example:

{
    "packages": "external/",
    "$rootRepository": "ckeditor/ckeditor5#master",
    "dependencies": {
        /* ... */
    },
    "presets": {
        "release": {
            "$rootRepository": "ckeditor/ckeditor5#release"
            /* ... */
        }
    }
}

Extra point: figure out how to combine the $rootRepository with presets.

The name of the option is a subject to discuss.

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