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.)
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.