Using Git for FreeBSD Development Today
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.
Git for FreeBSD Development in the Future
In 2019, Core chartered a Git Working Group (May 2019 DevSummit announcement (Youtube); Slides from same, p. 49+) to work out the implementation details of using Git as the primary VCS for FreeBSD development.