diff tests: don't ignore "git rev-list" exit code

diff tests: don't ignore "git rev-list" exit code

Change a fragile pattern introduced in 2b459b483cb (diff: make sure
work tree side is shown as 0{40} when different, 2008-03-02) to check
the exit code of "git rev-list", while we're at it let's get rid of
the needless sub-shell for invoking it in favor of the "-C" option.

Because of this I'd marked these tests as passing under SANITIZE=leak
in 16d4bd4f14e (leak tests: mark some diff tests as passing with
SANITIZE=leak, 2021-10-31), let's remove the
"TEST_PASSES_SANITIZE_LEAK=true" annotation as they no longer do.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>