stack: display 's#' instead of 't#' and 'b#' If we want people to use it, we should display it.

stack: clarify s# error message in case of branch A branch might have a lots of changeset, with all of them public.

alias: allow reference through 's#' We keep the 't#' option, but we hope to be able to deprecate it at some point. We move to 's#' because it refers to `stack` a more generic concept. than topic. In addition, this allows to phase out 'b#' that collide with normal hex-node.