t4041 (diff-submodule-option): modernize style

- Enclose tests in single quotes as opposed to double quotes.  This is
  the prevalent style in other tests.
- Remove the unused variable $head4_full.
- Indent the expected output so that it lines up with the rest of the
  test text.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: artagnon