feat(reference): add ability to compress references

lib/src/bindings/refdb.dart

@@ -0,0 +1,20 @@
+import 'dart:ffi';
+import '../error.dart';
+import 'libgit2_bindings.dart';
+import '../util.dart';
+
+/// Suggests that the given refdb compress or optimize its references.
+/// This mechanism is implementation specific. For on-disk reference databases,
+/// for example, this may pack all loose references.
+///
+/// Throws a [LibGit2Error] if error occured.
+void compress(Pointer<git_refdb> refdb) {
+  final error = libgit2.git_refdb_compress(refdb);
+
+  if (error < 0) {
+    throw LibGit2Error(libgit2.git_error_last());
+  }
+}
+
+/// Close an open reference database to release memory.
+void free(Pointer<git_refdb> refdb) => libgit2.git_refdb_free(refdb);

lib/src/reference.dart

@@ -4,6 +4,8 @@ import 'package:libgit2dart/libgit2dart.dart';
 import 'bindings/libgit2_bindings.dart';
 import 'bindings/reference.dart' as bindings;
 import 'bindings/object.dart' as object_bindings;
+import 'bindings/refdb.dart' as refdb_bindings;
+import 'bindings/repository.dart' as repository_bindings;
 import 'blob.dart';
 import 'commit.dart';
 import 'oid.dart';
@@ -40,6 +42,17 @@ class References {
     final refPointer = bindings.lookup(_repoPointer, name);
     return Reference(_repoPointer, refPointer);
   }
+
+  /// Suggests that the given refdb compress or optimize its references.
+  /// This mechanism is implementation specific. For on-disk reference databases,
+  /// for example, this may pack all loose references.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  void compress() {
+    final refdb = repository_bindings.refdb(_repoPointer);
+    refdb_bindings.compress(refdb);
+    refdb_bindings.free(refdb);
+  }
 }
 
 class Reference {

test/reference_test.dart

@@ -514,5 +514,17 @@ void main() {
       tree.free();
       ref.free();
     });
+
+    test('successfully compresses references', () {
+      final packedRefsFile = File('$tmpDir.git/packed-refs');
+      expect(packedRefsFile.existsSync(), false);
+      final oldRefs = repo.references.list();
+
+      repo.references.compress();
+
+      expect(packedRefsFile.existsSync(), true);
+      final newRefs = repo.references.list();
+      expect(newRefs, oldRefs);
+    });
   });
 }