t4002 (diff-basic): use test_might_fail for commands that might fail

t4002 (diff-basic): use test_might_fail for commands that might fail

Also replace '|| return 1' by '&&' to allow chain of operations to be
checked for proper return status, and modify the update-index command
as suggested by Jonathan Nieder to not exit early but try to make sure
files that match the work tree are marked as matching.

Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>