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.