gitsync is a tool that allows you to sync a git repository's remotes. It was designed to allow me to sync public repositories to a private CI system without having to manually check for changes and mirror them.
It can sync multiple branches per remote pair, and can be run inside and outside of a current working directory that contains a git repository, as long as the repo is valid on disk.
The release is only built for macOS. Look at build.sh to build it for yourself (go build isn't quite enough due to the LDFLAGS population).