git diff: rename test that had a conflicting name

In 86140d5 the new test t4041-diff-submodule.sh was introduced although t4027-diff-submodule.sh already existed. Rename the newer test to t4041-diff-submodule-option.sh to fix that. Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>