remote: rename inmemory to anonymous and swap url and fetch order

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.
Merge pull request #2206 from libgit2/cmn/inmemory-swap-order

Rename in-memory remote to anonymous and swap url and fetch order