Re: concurrent fetches to update same mirror

> Interestingly, in the case of ref _creation_, not update, like this:
>
>   mkdir repo && cd repo && git init
>   git remote add origin some-remote-repo-that-takes-a-few-seconds
>   xterm -e 'git fetch -v; read' & xterm -e 'git fetch -v; read'
>
> then both will happily update, the second one overwriting the results of
> the first. It seems in the case of locking a ref which previously didn't
> exist, we don't enforce that it still doesn't exist.