The current interface, if given a tweak, uses a *different secret key* and tweaks it. This was an early experiment: we will switch to using a secret tweak for invoice_requests like we do for invoice path ids. To make sure there's no funny business, *hsmd* hashes to form the tweak (i.e. no zero tweaks!). Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>