add name_to_id binding and headCommit to repo

- implemented git_reference_name_to_id binding
- created repository.headCommit getter

test/reference_test.dart

@@ -530,6 +530,11 @@ void main() {
       );
     });
 
+    test('get oid by name', () {
+      final oid = Reference.nameToId(repo: repo, refName: 'HEAD');
+      expect(oid, repo.head.target);
+    });
+
     test('manually releases allocated memory', () {
       final ref = Reference.lookup(repo: repo, name: 'refs/heads/master');
       expect(() => ref.free(), returnsNormally);

test/repository_test.dart

@@ -246,6 +246,10 @@ void main() {
       expect(() => repo.free(), returnsNormally);
     });
 
+    test('get head commit', () {
+      expect(repo.headCommit.oid.sha, lastCommit);
+    });
+
     test('returns string representation of Repository object', () {
       expect(repo.toString(), contains('Repository{'));
     });