update: allow dirty update to foreground (successors) Update to "foreground" are no longer seen as cross branch update. "Foreground" are descendants or successors (or successors of descendants (or descendant of successors (etc))). This allows to update with uncommited changes that get automatically merged. This changeset is a small step forward. We want to allow dirty update to "background" (precursors) and takes obsolescence in account when finding the default update destination. But those requires deeper changes and will comes in later changesets.