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>

t/t5318-commit-graph.sh

@@ -399,6 +399,12 @@ corrupt_graph_and_verify() {
 
 }
 
+test_expect_success 'detect too small' '
+	corrupt_graph_setup &&
+	echo "a small graph" >$objdir/info/commit-graph &&
+	corrupt_graph_verify "too small"
+'
+
 test_expect_success 'detect bad signature' '
 	corrupt_graph_and_verify 0 "\0" \
 		"graph signature"