Managing Drupal Core in Git for Aegir Platforms
Our current (simplified) practice at work:


Pull down the drupal core git repo http://drupalcode.org/project/drupal.git/
Add upstream to your local project that tracks the above repo
When a new version of drupal is released, fetch the drupal repo and merge the appropriate tag


e.g.

cd /var/www/your-platform/
git fetch upstream
git merge 7.7