The first version of infinitepush smartlog summary included some false
positives (in particular, not-backed-up but obsolete revs), and took a long
time to compute the set if there were many heads in the repo.
Improve the revset that is used by taking into account the heads that *should*
be backed up. Ancestors of these heads, that are not ancestors of the actual
backup heads are the ones that should be considered "not backed up".
Should we have and not obsolete() here?