In this case, the user's default was info, but they specifically asked for debug
from one plugin. Since there were no per-file filters, it set filtering to the
default level, info, and rejected it. Since it's been explicitly filtered in,
we need to pass it at this point.
Reported-by: @wtogami
Fixes: #6503
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>