apply changes properly when using multiple lowerdirs

[ezrizhu]

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 #122 (comment)