We used to consider it an error if a remote didn't have at least a
fetch refspec. This was too much checking, as a remote doesn't in fact
need to have anything other than an URL configured to be considered
a remote.
git_refspec_transform_r assumed that the reference name passed would
be only a branch or tag name. This is not the case, and we need to
take into consideration what's in the refspec's source to know how
much of the prefix to ignore.
