stabilize: improve unstable selection heuristic

Without argument, stabilize was picking the first in:

  "unstable() and ((suspended() or obsancestors(::.))::)"

which usually returned the "oldest" unstable revision in parent
predecessors descendants. This revision is interesting because it
usually gives "soft" merges but rebasing it left the working directory
on a remote branch, which was very confusing.

The new heuristic picks an unstable changeset which can be rebased on
top of the parent revision, or on top of one of its descendants
(selected in revision order). This has the advantage of selecting a
revision which can be rebased on the current subtree, and leave the
working directory in a more convenient location.