t/t5510: demonstrate failure to fetch when current branch has merge ref

t/t5510: demonstrate failure to fetch when current branch has merge ref

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:

   git checkout master
   git config branch.master.merge refs/heads/master
   git fetch git://git.kernel.org/pub/scm/git/git.git

but this one does:

   git config --unset branch.master.merge
   git fetch git://git.kernel.org/pub/scm/git/git.git

Add a test to demonstrate this flaw.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>