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.

topic-revset: get the list of revset through the official localrepo API This is simpler and faster. In addition we tighter the logic to be used in case of empty topic. We do not need to fall back to revision search.

topic-revset: strictly read string Making a clear distinction between string and symbol passed to topic is important when it comes to empty topic. Filtering an empty topic name is harmless. Searching a non existent revision is not.

test: search for empty topic as a string The reduce the ambiguity in the test to help with the next change.