git clone --recursive git://github.com/b00y0h/wordpress.git
cd wordpress
copy dev_settings_sample.php to dev_settings.php and edit with your db settings.
cp dev_settings_sample.php dev_settings.php
cd wordpress-core
git fetch --tags
git checkout 3.5.1
Replace 3.5.1 with the correct version number.
Now commit the changes subrepository version to your main project:
cd ..
git add wordpress-core
git commit -m "Update Wordpress to version 3.5.1"