prev: jump to parent's successor if parent is obsolete and topic is involved `hg stack` shows a linear chain of commits even when they are not linear chain and some changesets are obsoleted and have successors as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 However when `hg prev` is run on an unstable changeset whose parent is obsolete and does not topic but parents successor has the topic, it shows no parent on the topic.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.

#require test-repo

  $ checkcm() {
  >   if ! (which check-manifest > /dev/null); then
  >     echo skipped: missing tool: check-manifest;
  >     exit 80;
  >   fi;
  > };
  $ checkcm
  $ cat << EOF >> $HGRCPATH
  > [experimental]
  > evolution=all
  > EOF

Run check manifest:

  $ cd $TESTDIR/..
  $ check-manifest
  lists of files in version control and sdist match