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.