Deleted branch is recreated during mirroring, even though it doesn't exist upstream anymore
Description of the problem
https://gitlab.com/triqng/triq is set to mirror from https://github.com/triqng/triq, and it works well, but there's something wrong with how branches are handled. Back when the mirror was set up, there used to be a stale branch named release-0.1.0, which has since been deleted. For some reason, the branch gets recreated on each mirror run, even though there's no such branch upstream anymore. I've also tried deleting the branch via gitlab.com's web interface, but it reappears on the next run.