feat(commit): add base bindings and api

lib/src/commit.dart

@@ -0,0 +1,78 @@
+import 'dart:ffi';
+import 'bindings/libgit2_bindings.dart';
+import 'bindings/commit.dart' as bindings;
+import 'oid.dart';
+import 'signature.dart';
+import 'util.dart';
+
+class Commit {
+  /// Initializes a new instance of [Commit] class from provided pointer to
+  /// commit object in memory.
+  ///
+  /// Should be freed with `free()` to release allocated memory.
+  Commit(this._commitPointer) {
+    libgit2.git_libgit2_init();
+  }
+
+  /// Initializes a new instance of [Commit] class from provided pointer to [Repository]
+  /// object in memory and pointer to [Oid] object in memory.
+  ///
+  /// Should be freed with `free()` to release allocated memory.
+  Commit.lookup(Pointer<git_repository> repo, Pointer<git_oid> oid) {
+    libgit2.git_libgit2_init();
+    _commitPointer = bindings.lookup(repo, oid);
+  }
+
+  /// Pointer to memory address for allocated commit object.
+  late final Pointer<git_commit> _commitPointer;
+
+  /// Returns the encoding for the message of a commit, as a string
+  /// representing a standard encoding name.
+  String get messageEncoding => bindings.messageEncoding(_commitPointer);
+
+  /// Returns the full message of a commit.
+  ///
+  /// The returned message will be slightly prettified by removing any potential leading newlines.
+  String get message => bindings.message(_commitPointer);
+
+  /// Returns the id of a commit.
+  Oid get id => Oid(bindings.id(_commitPointer));
+
+  /// Returns the commit time (i.e. committer time) of a commit.
+  int get time => bindings.time(_commitPointer);
+
+  /// Returns the committer of a commit.
+  Signature get committer => Signature(bindings.committer(_commitPointer));
+
+  /// Returns the author of a commit.
+  Signature get author => Signature(bindings.author(_commitPointer));
+
+  /// Returns list of parent commits.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  List<Commit> get parents {
+    var parents = <Commit>[];
+    final parentCount = bindings.parentCount(_commitPointer);
+
+    for (var i = 0; i < parentCount; i++) {
+      final parentOid = bindings.parentId(_commitPointer, i);
+
+      if (parentOid != nullptr) {
+        final owner = bindings.owner(_commitPointer);
+        final commit = bindings.lookup(owner, parentOid);
+        parents.add(Commit(commit));
+      }
+    }
+
+    return parents;
+  }
+
+  /// Get the id of the tree pointed to by a commit.
+  Oid get tree => Oid(bindings.tree(_commitPointer));
+
+  /// Releases memory allocated for commit object.
+  void free() {
+    bindings.free(_commitPointer);
+    libgit2.git_libgit2_shutdown();
+  }
+}