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.

#require test-repo pyflakes

Copied from Mercurial core (60ee2593a270)

  $ cd "`dirname "$TESTDIR"`"

run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)

  $ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
  > | xargs pyflakes 2>/dev/null