Freeze string literals

See Danger's suggestions:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6869#note_93730253

app/policies/commit_policy.rb

+# frozen_string_literal: true
+
 class CommitPolicy < BasePolicy
   delegate { @subject.project }
 end

spec/features/commits/user_uses_slash_commands_spec.rb

+# frozen_string_literal: true
+
 require 'rails_helper'
 
 describe 'Commit > User uses quick actions', :js do

spec/services/commits/tag_service_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Commits::TagService do

spec/services/quick_actions/target_service_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe QuickActions::TargetService do