Shortcut already-hardlinked files

If an inode in the Nix store has more than 1 link, it probably means that it was linked into .links/ by us. If so, skip. This results in super-fast optimization runs.

There's a possibility that something else hardlinked the file, so it would be nice to be able to override this.

Note: by looking at the number of hardlinks for each of the files in .links/, you can get accurate deduplication numbers and space savings. This requires going through all files in the .links/ directory, but that may be a good idea anyway, to remove the ones that have only one link.