prev: fix the breakage of `hg previous` from obsolete cset This patch fix the breakage of `hg prev` from obsolete changeset when topic is involved. We get the topicidx of it's succesor and proceed.

prev: add a test case which shows traceback when doing prev on obsolete cset This patch adds a test which shows how `hg prev` is borken when called from a obsolete changeset and there is an active topic. The next patch will fix that.