(optimization) Reuse non-changed (region) files when restoring.

[Alvinn8]

Large worlds that are being restored will take a long time to restore from mch objects, and chances are many region files will be unchanged since the commit in question, for example, chunks that were explored to find a woodland mansion or something and then never touched again.

What we can do when restoring is read the current live state of the world, and if the region file has not been changed since the commit we want to restore to, we can just download/open that region file and copy it as-is, without having to parse it which should save quite a bit of time.

[Alvinn8]

Investigate when/if this would be faster. I was able to restore a 150 MB world in only a few seconds completely from mch storage. When the files are from a remote it will probably just be slower to download them from the remote than to create them locally.