Make "s3sync sync foo bucket:bar" idempotent

Previously, running "s3sync foo bucket:bar" twice in a row, regardless of
whether the contents of "foo" had changed, would result in the second run
uploading and then deleting the same files, leaving you with an empty S3
bucker:folder.

The other direction, "s3sync sync bucket:bar foo", remains idempotent, if a
little wonky.