prev: jump to parent's successor if parent is obsolete and topic is involved `hg stack` shows a linear chain of commits even when they are not linear chain and some changesets are obsoleted and have successors as separate head. This is very nice to have a post evolution view of the stack. However when `hg prev` is run on an unstable changeset whose parent is obsolete and does not topic but parents successor has the topic, it shows no parent on the topic. This patch makes `hg prev` update to parent's successor and make things follow the stack order.

#!/bin/sh
. $TESTDIR/testlib/pythonpath.sh

# This file holds logic that is used in many tests.
# It can be called in a test like this:
#  $ . "$TESTDIR/testlib/topic_setup.sh"

# Enable obsolete markers and enable extensions
cat >> $HGRCPATH << EOF
[experimental]
evolution=createmarkers,exchange

[extensions]
rebase=
topic=
EOF