WP Source Control is a WordPress plugin that allows you to source control your theme directory and your posts/pages. You can even see how your theme has changed since your last commit using the diff function.
1) Install WordPress 3.4.2 or higher
2) Download the following file: http://downloads.wordpress.org/plugin/wp-source-control.1.0.zip
3) Login to WordPress admin, click on Plugins / Add New / Upload, then upload the zip file you just downloaded.
4) Activate the plugin.