t/t5510-fetch.sh: improve testing with explicit URL and merge spec

t/t5510-fetch.sh: improve testing with explicit URL and merge spec

Commit 6106ce46 introduced a test to demonstrate fetch's failure to
retrieve any objects or update FETCH_HEAD when it was supplied a repository
URL and the current branch had a configured merge spec.  This commit
expands the original test based on comments from Junio Hamano.  In addition
to actually verifying that the fetch updates FETCH_HEAD correctly, and does
not update the current branch, two more tests are added to ensure that the
merge configuration is ignored even when the supplied URL matches the URL
of the remote configured for the branch.

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