stack: check if stack is empty more pythonically

stack: implement __bool__ and __nonzero__

stack: get stack data directly from stack and remove stackdata() stackdata function began its life in 137f8b04901e as a proto-stack: it computed stack revs on its own and only had one property to return, "changesetcount". Later it started to prepare and return more properties, but since b933a8068c17 it was computing revs using a getstack function. And then finally in 17749d9d3968 it started to rely on stack class entirely. It was a good function, but now, when all the logic it provided was factored into stack class, I'd say it's finally time for it to be put to rest.

stack: use stack._revs instead of stack.revs[1:] in external children revset The revset in question excludes all revs that are part of the stack. Using stack.revs[1:] works (rev #0 is stack base, which is not a part of the stack), but using stack._revs is technically more correct, because this variable is specifically designed to hold only revisions that are part of the stack.

test: update output to match the parent changesets This amend was forgotten.

topic: add a simple option to reject publishing The option is pretty basic but it can be used as base to build larger feature. The main target for going in this direction is to be able to distinct between user that are "simple contributors" pushing topic for review and the "maintainers" or "automation" that can publish changesets.