Relax the overlapping restriction of the backup-dir option

I’d like to propose relaxing the restriction of the backup-dir that it can’t overlap on the target remote. Instead allow the overlap but require that that backup-dir gets added as a “exclude” instead?

This would allow easier implementation of this feature without having to reorganize the remote to accommodate it. In order to effectively use it now, we need to move the hierarchy down a directory to accommodate the backup files being outside of the target we are syncing to. I use lots of mounts and changing the hierarchy to accommodate it now is difficult.