Create space-saving clones on macOS with Python

There are circumstances where having a reflink that doesn't take up space could be problematic or result in unexpected behaviour. An example: you have a huge file, e.g. databases or a huge "big data" dataset, maybe it's a terabyte or something, but it's on a disk that only has 500 GB free.

Now you want to do a nightly backup. With reflinks you `cp` the file and it succeeds despite not having enough space on the disk. Then your system starts up during the day, loads up the data set, and starts making changes; you zero-out the dataset and then start re-importing the data. Now because you're writing new blocks, your re-import fails halfway through because the disk is full.

Without reflinks, your nightly copy fails and you get an alert about it, and then you go clean it up.

This is kind of an idiotic, contrived example, but it's an example of the kind of thing that could happen with reflinks that wouldn't happen otherwise.