refactor

- add object-hash test

Co-authored-by: chatgpt-codex-connector[bot] <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>

Author: Byron

gix-commitgraph/tests/commitgraph.rs

@@ -81,6 +81,14 @@ pub fn graph_and_expected_named(
     let expected = inspect_refs(&repo_dir, refs);
     let cg =
         Graph::from_info_dir(&repo_dir.join(".git").join("objects").join("info")).expect("graph present and valid");
+    let object_hash = cg.object_hash();
+    if let Some(any_ref) = expected.values().next() {
+        assert_eq!(
+            object_hash,
+            any_ref.id().kind(),
+            "graph hash kind should match fixture object IDs"
+        );
+    }
     (cg, expected)
 }