add a git service type
because of how the platform works, you need a private git repository in order to effectively use the platform with more than one person.
but most people don't have a private git repo lying around.
we should add a service for 'git' that installs a minimal gitolite setup, with gitolite-admin access granted to the users configured in the platform.
(from redmine: created on 2014-03-23)