[Data Liberation] Edit static files directly in a git repository

[adamziel]

Motivation for the change, related issues

🚧 Experimental! Do not merge! 🚧

Experimenting for fun with editing WP_Filesystem files and implementing a WP_Git_Filesystem class.

$fs = new WP_Git_Filesystem(
    new WP_Git_Client(GIT_REPO_URL, [
        'author' => 'Adam Zieliński <adam@example.net>',
        'committer' => 'Adam Zieliński <adam@example.net>',
    ]),
    'main',
    '/pages'
);

// Every $fs operation creates a commit
$fs->put_contents('/index.md', '# Markdown file!' );
$fs->mkdir('/setup');
$fs->put_contents('/setup/01-introduction.md', '# Setup introduction' );
$fs->put_contents('/setup/02-installation.md', '# Install things' );
$fs->rmdir('/setup', ['recursive' => true']);

// In the future we could have a local index and batch all the
// updates to push as a single commit

[adamziel]

I'll close this in favor of a rebased and slightly cleaned up #2118