feat(reference): add ability to create direct reference

test/odb_test.dart

@@ -0,0 +1,46 @@
+import 'dart:io';
+
+import 'package:test/test.dart';
+import 'package:libgit2dart/src/repository.dart';
+import 'package:libgit2dart/src/odb.dart';
+import 'package:libgit2dart/src/oid.dart';
+
+import 'helpers/util.dart';
+
+void main() {
+  const lastCommit = '78b8bf123e3952c970ae5c1ce0a3ea1d1336f6e8';
+
+  group('Odb', () {
+    late Repository repo;
+    final tmpDir = '${Directory.systemTemp.path}/odb_testrepo/';
+
+    setUpAll(() async {
+      if (await Directory(tmpDir).exists()) {
+        await Directory(tmpDir).delete(recursive: true);
+      }
+      await copyRepo(
+        from: Directory('test/assets/testrepo/'),
+        to: await Directory(tmpDir).create(),
+      );
+      repo = Repository.open(tmpDir);
+    });
+
+    tearDownAll(() async {
+      repo.close();
+      await Directory(tmpDir).delete(recursive: true);
+    });
+
+    test('successfully initializes', () {
+      expect(repo.odb, isA<Odb>());
+      repo.odb.free();
+    });
+
+    test('finds object by short oid', () {
+      final shortSha = '78b8bf';
+      final shortOid = Oid.fromSHAn(shortSha);
+      final oid = repo.odb.existsPrefix(shortOid.pointer, shortSha.length);
+      expect(Oid(oid).sha, lastCommit);
+      repo.odb.free();
+    });
+  });
+}