You can clone with HTTPS, or Subversion.
Each version of Pressflow is API-compatible with the same major Drupal version. For example, Pressflow 6 is compatible with all Drupal 6 modules. Pressflow 6 also integrates the SimpleTest system from Drupal 7 and the CDN support patch.
http://pressflow.org/
Merge pull request #65 from pressflow/6.28 …
Drupal 6.28