feat(diff): add ability to apply diff

lib/src/bindings/diff.dart

@@ -267,6 +267,32 @@ Pointer<git_buf> addToBuf(Pointer<git_patch> patch, Pointer<git_buf> buffer) {
   }
 }
 
+/// Apply a diff to the given repository, making changes directly in the working directory,
+/// the index, or both.
+///
+/// Throws a [LibGit2Error] if error occured.
+bool apply(
+  Pointer<git_repository> repo,
+  Pointer<git_diff> diff,
+  int location, [
+  bool check = false,
+]) {
+  final opts = calloc<git_apply_options>();
+  libgit2.git_apply_options_init(opts, GIT_APPLY_OPTIONS_VERSION);
+  if (check) {
+    opts.ref.flags = git_apply_flags_t.GIT_APPLY_CHECK;
+  }
+  final error = libgit2.git_apply(repo, diff, location, opts);
+
+  calloc.free(opts);
+
+  if (error < 0) {
+    return check ? false : throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return true;
+  }
+}
+
 /// Free a previously allocated diff stats.
 void statsFree(Pointer<git_diff_stats> stats) =>
     libgit2.git_diff_stats_free(stats);

lib/src/git_types.dart

@@ -1046,3 +1046,29 @@ class GitDiffLine {
   @override
   String toString() => 'GitDiffLine.$_name';
 }
+
+/// Possible application locations for `apply()`
+class GitApplyLocation {
+  const GitApplyLocation._(this._value, this._name);
+  final int _value;
+  final String _name;
+
+  /// Apply the patch to the workdir, leaving the index untouched.
+  /// This is the equivalent of `git apply` with no location argument.
+  static const workdir = GitApplyLocation._(0, 'workdir');
+
+  /// Apply the patch to the index, leaving the working directory
+  /// untouched. This is the equivalent of `git apply --cached`.
+  static const index = GitApplyLocation._(1, 'index');
+
+  /// Apply the patch to both the working directory and the index.
+  /// This is the equivalent of `git apply --index`.
+  static const both = GitApplyLocation._(2, 'both');
+
+  static const List<GitApplyLocation> values = [workdir, index, both];
+
+  int get value => _value;
+
+  @override
+  String toString() => 'GitApplyLocation.$_name';
+}

lib/src/repository.dart

@@ -798,4 +798,25 @@ class Repository {
   }) {
     return a.diff(newBlob: b, oldAsPath: aPath, newAsPath: bPath);
   }
+
+  /// Applies the [diff] to the given repository, making changes directly in the working directory.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void apply(Diff diff) {
+    diff_bindings.apply(
+      _repoPointer,
+      diff.pointer,
+      GitApplyLocation.workdir.value,
+    );
+  }
+
+  /// Checks if the [diff] will apply to HEAD.
+  bool applies(Diff diff) {
+    return diff_bindings.apply(
+      _repoPointer,
+      diff.pointer,
+      GitApplyLocation.index.value,
+      true,
+    );
+  }
 }

test/diff_test.dart

@@ -194,6 +194,22 @@ index e69de29..c217c63 100644
       diff.free();
     });
 
+    test(
+        'checks if diff can be applied to repository and successfully applies it',
+        () {
+      final diff = Diff.parse(patchText);
+      final file = File('${tmpDir}subdir/modified_file');
+
+      repo.reset('a763aa560953e7cfb87ccbc2f536d665aa4dff22', GitReset.hard);
+      expect(file.readAsStringSync(), '');
+
+      expect(repo.applies(diff), true);
+      repo.apply(diff);
+      expect(file.readAsStringSync(), 'Modified content\n');
+
+      diff.free();
+    });
+
     test('successfully creates patch from entry index in diff', () {
       final diff = Diff.parse(patchText);
       final patch = Patch.fromDiff(diff, 0);