test-tool: use 'read-graph' helper

The 'git commit-graph read' subcommand is used in test scripts to check
that the commit-graph contents match the expected data. Mostly, this
helps check the header information and the list of chunks. Users do not
need this information, so move the functionality to a test helper.

Reported-by: Bryan Turner <bturner@atlassian.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 115aabd..c248234 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -25,7 +25,7 @@
 	num_commits: $1
 	chunks: oid_fanout oid_lookup commit_metadata
 	EOF
-	git commit-graph read >output &&
+	test-tool read-graph >output &&
 	test_cmp expect output
 }