refactor(commit)!: return Tree object instead of Oid for tree getter

lib/src/commit.dart

@@ -1,6 +1,7 @@
 import 'dart:ffi';
 import 'bindings/libgit2_bindings.dart';
 import 'bindings/commit.dart' as bindings;
+import 'bindings/tree.dart' as tree_bindings;
 import 'repository.dart';
 import 'oid.dart';
 import 'signature.dart';
@@ -98,8 +99,12 @@ class Commit {
     return parents;
   }
 
-  /// Get the id of the tree pointed to by a commit.
+  /// Get the tree pointed to by a commit.
-  Oid get tree => Oid(bindings.tree(_commitPointer));
+  Tree get tree {
+    final repo = bindings.owner(_commitPointer);
+    final oid = bindings.tree(_commitPointer);
+    return Tree(tree_bindings.lookup(repo, oid));
+  }
 
   /// Releases memory allocated for commit object.
   void free() => bindings.free(_commitPointer);

test/checkout_test.dart

@@ -44,7 +44,7 @@ void main() {
     test('successfully checkouts tree', () {
       final masterHead =
           repo['821ed6e80627b8769d170a293862f9fc60825226'] as Commit;
-      final masterTree = repo[masterHead.tree.sha] as Tree;
+      final masterTree = masterHead.tree;
       expect(
         masterTree.entries.any((e) => e.name == 'another_feature_file'),
         false,
@@ -53,7 +53,7 @@ void main() {
       repo.checkout(refName: 'refs/heads/feature');
       final featureHead =
           repo['5aecfa0fb97eadaac050ccb99f03c3fb65460ad4'] as Commit;
-      final featureTree = repo[featureHead.tree.sha] as Tree;
+      final featureTree = featureHead.tree;
       final repoHead = repo.head;
       expect(repoHead.target.sha, featureHead.id.sha);
       expect(repo.status, isEmpty);

test/commit_test.dart

@@ -74,7 +74,7 @@ void main() {
       expect(commit.author, author);
       expect(commit.committer, commiter);
       expect(commit.time, 124);
-      expect(commit.tree.sha, tree);
+      expect(commit.tree.id.sha, tree);
       expect(commit.parents.length, 1);
       expect(commit.parents[0].sha, mergeCommit);
 
@@ -99,7 +99,7 @@ void main() {
       expect(commit.author, author);
       expect(commit.committer, commiter);
       expect(commit.time, 124);
-      expect(commit.tree.sha, tree);
+      expect(commit.tree.id.sha, tree);
       expect(commit.parents.length, 0);
 
       commit.free();
@@ -123,7 +123,7 @@ void main() {
       expect(commit.author, author);
       expect(commit.committer, commiter);
       expect(commit.time, 124);
-      expect(commit.tree.sha, tree);
+      expect(commit.tree.id.sha, tree);
       expect(commit.parents.length, 2);
       expect(commit.parents[0].sha, mergeCommit);
       expect(commit.parents[1].sha, 'fc38877b2552ab554752d9a77e1f48f738cca79b');
@@ -149,7 +149,7 @@ void main() {
       expect(commit.author, author);
       expect(commit.committer, commiter);
       expect(commit.time, 124);
-      expect(commit.tree.sha, tree);
+      expect(commit.tree.id.sha, tree);
       expect(commit.parents.length, 1);
       expect(commit.parents[0].sha, mergeCommit);
 

test/merge_test.dart

@@ -183,9 +183,9 @@ void main() {
         final baseCommit =
             repo[repo.mergeBase(ourCommit.id.sha, theirCommit.id.sha).sha]
                 as Commit;
-        final theirTree = repo[theirCommit.tree.sha] as Tree;
+        final theirTree = theirCommit.tree;
-        final ourTree = repo[ourCommit.tree.sha] as Tree;
+        final ourTree = ourCommit.tree;
-        final ancestorTree = repo[baseCommit.tree.sha] as Tree;
+        final ancestorTree = baseCommit.tree;
 
         final mergeIndex = repo.mergeTrees(
           ancestorTree: ancestorTree,
@@ -221,9 +221,9 @@ void main() {
         final baseCommit =
             repo[repo.mergeBase(ourCommit.id.sha, theirCommit.id.sha).sha]
                 as Commit;
-        final theirTree = repo[theirCommit.tree.sha] as Tree;
+        final theirTree = theirCommit.tree;
-        final ourTree = repo[ourCommit.tree.sha] as Tree;
+        final ourTree = ourCommit.tree;
-        final ancestorTree = repo[baseCommit.tree.sha] as Tree;
+        final ancestorTree = baseCommit.tree;
 
         final mergeIndex = repo.mergeTrees(
           ancestorTree: ancestorTree,

test/reference_test.dart

@@ -502,7 +502,7 @@ void main() {
     test('successfully peels to object of provided type', () {
       final ref = repo.references['refs/heads/master'];
       final commit = repo[ref.target.sha] as Commit;
-      final tree = repo[commit.tree.sha] as Tree;
+      final tree = commit.tree;
       final peeledCommit = ref.peel(GitObject.commit) as Commit;
       final peeledTree = ref.peel(GitObject.tree) as Tree;