git diff: rename test that had a conflicting name

In 86140d56 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: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>