checkout: Set the file mode

Dart unfortunately doesn't provide a way, however this convenient
package 'os' allows us to change it via FFI.

Related to dart-lang/sdk#15078

lib/checkout.dart

@@ -1,4 +1,7 @@
+import 'dart:io';
+
 import 'package:path/path.dart' as p;
+import 'package:os/file_system.dart' as os_fs;
 
 import 'package:dart_git/dart_git.dart';
 import 'package:dart_git/diff_commit.dart';
@@ -128,7 +131,10 @@ extension Checkout on GitRepository {
         await fs
             .directory(p.join(workTree, p.dirname(to.path)))
             .create(recursive: true);
-        await fs.file(p.join(workTree, to.path)).writeAsBytes(blobObj.blobData);
+
+        var filePath = p.join(workTree, to.path);
+        await fs.file(filePath).writeAsBytes(blobObj.blobData);
+        os_fs.chmodSync(File(filePath), change.to!.mode.val);
 
         await index.updatePath(to.path, to.hash);
       } else if (change.deleted) {

pubspec.yaml

@@ -13,6 +13,8 @@ dependencies:
   file: ^6.1.0
   freezed_annotation: ^0.14.1
   meta: ^1.3.0
+  os:
+    git: https://github.com/GitJournal/os.git
   path: ^1.8.0
   quiver: ^3.0.0
   tuple: ^2.0.0