apply changes properly when using multiple lowerdirs

> Talking with @angelhof about this, we realized there's a tricky interaction with `process_changes` when you have multiple lowerdirs: you have to replay the merge to do the comparison.
> The correct solution is to mergerfs mount the layers of lowerdir and compare against _that_ merged lowerdir (rather than the `$local_file` which just absolutizes to the real system root).
> But until we have a use case where someone would want to use summaries and multiple lowerdirs (`hs` doesn't want the summaries), we should punt on this and give a warning that `-L` implies `-n`.

_Originally posted by @mgree in https://github.com/binpash/try/issues/122#issuecomment-1772732770_