With opportunistic ref updates, git has introduced the concept of having
base refspecs *and* refspecs that are active for a particular fetch.
Let's start by letting the user override the refspecs for download.
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.
