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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jlehmann