Source Depot. Forked years ago with tons of added features. Various Halo titles also used it and had easy to use integrations with most of the art and design pipeline tools.
It was stated in Saeed Noursalehi's talk "Scaling Git at Microsoft", held at Git-Merge 2017. Until the conference recordings are available, here is the closest thing to a "source": https://twitter.com/no_more_ducks/status/827479795185364993
I seem to recall that Microsoft has previously used a custom Perforce "fork" for their larger code bases (Windows, Server, Office, etc.).