stack: rename stack.revs into stack.indexedrevs They are accessible by an index, and are sorted. But they don't include all revisions for show in showstack().

stack: return from behindcount() early Reduces indentation.

stack: use self._revs in stack.behindcount() This code also doesn't care about the order of revisions, it just wants all revs except stack base, and that's what self._revs is.