commit-graph tests: fix unportable "dd" invocation

commit-graph tests: fix unportable "dd" invocation

Change an unportable invocation of "dd" with count=0, that wanted to
truncate the commit-graph file.  In POSIX it is unspecified what
happens when count=0 is provided[1]. The NetBSD "dd" behavior
differs from GNU (and seemingly other BSDs), which has left this test
broken since d2b86fbaa1 ("commit-graph: fix buffer read-overflow",
2019-01-15).

Copying from /dev/null would seek/truncate to seek=$zero_pos and
stop immediately after that (without being able to copy anything),
which is the right way to truncate the file.

1. http://pubs.opengroup.org/onlinepubs/9699919799/utilities/dd.html

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