config: have a way to backup and restore value in config This is introduce to allow temporary overwriting of a config value while being able to reinstall the old value once done. The main advantage over using ``config`` and ``setconfig`` is that backup and restore will properly restore the lack of any config. Restoring the fact that there was no value is important to allow config user to keep using meaniful default value. A more naive approach will result in the following scenario:: Before: config(section, item, my_default) --> my_default temporal overwrite old = config(section, item) ? setconfig(section, item, old) After config(section, item, my_default) --> None The first user of this feature should be mq to overwriting minimal phase of future commit.