PostReceive: Don't generate push_data unless necessary

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31641#note_202201578, for each tag or branch pushed, it looks like we request a CommitDelta to generate the push data. From BaseHooksService, we run:

    def execute_project_hooks
      project.execute_hooks(push_data, hook_name)
      project.execute_services(push_data, hook_name)
    end

Project#execute_hooks will run for both project-specific hooks and system hooks. We could avoid all this work by optimizing this so that:

1. Only execute hooks if there are any relevant hooks
2. Only execute services if there are any relevant services