When 'git fetch' is supplied just a repository URL (not a remote name),
and without a fetch refspec, it should fetch from the remote HEAD branch
and update FETCH_HEAD with the fetched ref. Currently, when 'git fetch'
is called like this, it fails to retrieve anything, and does not update
FETCH_HEAD, if the current checked-out branch has a configured merge ref.
i.e. this fetch fails to retrieve anything nor update FETCH_HEAD: