commit-graph tests: test a graph that's too small

commit-graph tests: test a graph that's too small

Use the recently split-up components of the corrupt_graph_and_verify()
function to assert that we error on graphs that are too small. The
error was added in 2a2e32bdc5 ("commit-graph: implement git
commit-graph read", 2018-04-10), but there was no test for it.

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