stack: handle external-children just like other states This also fixes color labels: previously when external-children was combined with another state, --color=debug would show something like: [topic.stack.index topic.stack.index.current - external-children|s4] Now there is a separate label for each state.

stack: optimize revset used for stack --children Since that option needs to exclude changesets on the current stack, let's just use stack.revs (that are cached) for that and skip branch() or topic() revset. st.revs[0] is the stack base, we don't consider it actual part of stack.

stack: rename unstable -> orphan That's the only instability stack can currently show, but it's about to show more and showing "unstable" for one type of instability just isn't good.