Set up a Git version control repository that integrates with OpenAtrium
Check out the "What theme and modules does Drupal.org use?" page.

Specifically for Git repository management, you'll want to use this set of modules:


http://drupal.org/project/versioncontrol
http://drupal.org/project/versioncontrol_git


And if you want a project release / issue tracking section like drupal.org, see:


http://drupal.org/project/project
http://drupal.org/project/project_issue
http://drupal.org/project/versioncontrol_project