t4041 (diff-submodule-option): parse digests sensibly

`git rev-list --max-count=1 HEAD` is a roundabout way of saying `git
rev-parse --verify HEAD`; replace a bunch of instances of the former
with the latter.  Also, don't unnecessarily `cut -c1-7` the rev-parse
output when the `--short` option is available.

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