commit-graph: test verifying a corrupt v2 header

The commit-graph file format v2 changes the v1 data only in the
header information. Add tests that check the 'verify' subcommand
catches corruption in the v2 header.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

t/t5318-commit-graph.sh

@@ -497,6 +497,37 @@ test_expect_success 'git fsck (checks commit-graph)' '
 	test_must_fail git fsck
 '
 
+test_expect_success 'rewrite commmit-graph with version 2' '
+	rm -f .git/objects/info/commit-graph &&
+	git commit-graph write --reachable --version=2 &&
+	git commit-graph verify
+'
+
+GRAPH_BYTE_CHUNK_COUNT=5
+GRAPH_BYTE_REACH_INDEX=6
+GRAPH_BYTE_UNUSED=7
+GRAPH_BYTE_HASH=8
+
+test_expect_success 'detect low chunk count (v2)' '
+	corrupt_graph_and_verify $GRAPH_CHUNK_COUNT "\02" \
+		"missing the .* chunk"
+'
+
+test_expect_success 'detect incorrect reachability index' '
+	corrupt_graph_and_verify $GRAPH_REACH_INDEX "\03" \
+		"reachability index version"
+'
+
+test_expect_success 'detect non-zero unused byte' '
+	corrupt_graph_and_verify $GRAPH_BYTE_UNUSED "\01" \
+		"unsupported value"
+'
+
+test_expect_success 'detect bad hash version (v2)' '
+	corrupt_graph_and_verify $GRAPH_BYTE_HASH "\00" \
+		"hash algorithm"
+'
+
 test_expect_success 'setup non-the_repository tests' '
 	rm -rf repo &&
 	git init repo &&