GitRepository.fetch loops forever

Discovered this while running `git skara backport --from <remote-address> <sha>` with a <remote-address> that was, by my mistake, a different repository. The behavior was that I saw a message "Fetching ..." forever. While of course I made a mistake, the skara tooling should exit when the underlying git command exits with an error, and propagate the message to the user, I think.