prev: jump to parent's successor if parent is obsolete and topic is involved
`hg stack` shows a linear chain of commits even when they are not linear chain
and some changesets are obsoleted and have successors as separate head. This is
very nice to have a post evolution view of the stack. However when `hg prev` is
run on an unstable changeset whose parent is obsolete and does not topic but
parents successor has the topic, it shows no parent on the topic.
This patch makes `hg prev` update to parent's successor and make things follow
the stack order.