Case in point. Emacs moved from bzr to git, and they did it like that.
Official Git repo: https://savannah.gnu.org/git/?group=emacs
Unofficial github mirror repo: https://github.com/emacs-mirror/emacs
Official Git repo: https://savannah.gnu.org/git/?group=emacs
Unofficial github mirror repo: https://github.com/emacs-mirror/emacs
https://github.com/gcc-mirror/gcc
It seems out of sync though.