There is no reason why we need to use a callback here. A string array
fits better with the usage, as this is not an event and we don't need
anything from the user.
The order in this function is the opposite to what
create_with_fetchspec() has, so change this one, as url-then-refspec is
what git does.
As we need to break compilation and the swap doesn't do that, let's take
this opportunity to rename in-memory remotes to anonymous as that's
really what sets them apart.
