prev: jump to parent's successor if parent is obsolete and topic is involved `hg stack` shows a linear chain of commits even when they are not linear chain and some changesets are obsoleted and have successors as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 However when `hg prev` is run on an unstable changeset whose parent is obsolete and does not topic but parents successor has the topic, it shows no parent on the topic.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.

tests: add a test showing wrong behaviour doing `hg prev` on orphan cset If there is an unstable changeset in the topic stack, whose parent is not a part of stack but has a successor which is part of stack, hg stack does shows them in linear order, but `hg prev` on that orphan changset says no parent on that topic. However it should update to the successor of the changeset. It will be fixed in the next patch.