t3508: add check_head_differs_from() helper function and use it

t3508: add check_head_differs_from() helper function and use it

In a test like:

test "$(git rev-parse --verify HEAD)" != "$(git rev-parse --verify fourth)"

the --verify does not accomplish much, since the exit status of
git rev-parse is not propagated to test. So it is more robust to
define and use the helper functions check_head_differs_from() and
check_head_equals() as done by this patch.

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>