Here are references and notes about using Git for FreeBSD development.
(editor's note: topics such as which source control system to use can tend to be religious in nature -- especially when it comes to the idea of migrating any and all FreeBSD repositories. Please keep this in mind as you read the following pages.)
fabient@ has written up some strategies for working with the Gitorious mirror.
The official endorsed way of using git to hack on FreeBSD is documented here: GitWorkflow.
The FreeBSD project page on Github has the official mirrors for src, doc, and ports repositories.
The Gitorious mirror does include a limited set of SVN branches, but JonathanAnderson's opinion is, "for the sake of a couple hundred extra MB (an extra 40% or so), why not carry around the complete revision history of the project *and* all branches, including releases, vendor branches and user stuff?"