commit-graph tests: fix error-hiding graph_git_two_modes() helper

commit-graph tests: fix error-hiding graph_git_two_modes() helper

The graph_git_two_modes() helper added in 177722b3442 (commit:
integrate commit graph with commit parsing, 2018-04-10) didn't
&&-chain its "git commit-graph" invocations, which as can be seen with
SANITIZE=leak will happily mark tests as passing if both of these
commands die, since test_cmp() will be comparing two empty files.

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