previous: fix behavior on obsolete cset when topic is involved (issue5708)
This patch fixes the behavior of `hg previous` when done on an obsolete
changeset having a successor of different topic.
The underlying code is bit messed up, I wanted to split the topic handling to
topic extension but I have postponed it to next bug.
previous: add test case when it's broken on obsolete cset
This patch adds couple of tests showing `hg previous` is broken on obsolete
changeset when the successor has different topic. Upcoming patch will fix this.