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.

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t