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.

There's a possibility that something else hardlinked the file, so it would be nice to be able to override this.

Also, by looking at the number of hardlinks for each of the files in .links/, you can get deduplication numbers and space savings.