Add missing third argument to `Git::Repository#initialize` in spec

spec/lib/gitlab/git/repository_spec.rb

@@ -980,7 +980,7 @@ describe Gitlab::Git::Repository, seed_helper: true do
     context 'with local and remote branches' do
       let(:repository) do
-        Gitlab::Git::Repository.new('default', File.join(TEST_MUTABLE_REPO_PATH, '.git'))
+        Gitlab::Git::Repository.new('default', File.join(TEST_MUTABLE_REPO_PATH, '.git'), '')
       end
       before do