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>

diff --git a/t/t4041-diff-submodule.sh b/t/t4041-diff-submodule-option.sh
similarity index 100%
rename from t/t4041-diff-submodule.sh
rename to t/t4041-diff-submodule-option.sh