repository_update_remote_mirror jobs never finish

I don't know how to reproduce this issue but I have observed in Sidekiq that a repository_update_remote_mirror job is created which idles and never finishes. Over time, more jobs get created so they have to be manually killed. At one point all our job slots were taken by these jobs which caused jobs to queue up before we restarted the server.

This ticket is quite vague but I suspect it can be fixed by a careful reading of the repository_update_remote_mirror worker code to find the situation where the job will never end.