exchange: check actually missing revs for obsolete / unstable revs (issue6372) The previous code was using `outgoing.ancestorsof`, which was originally called `outgoing.missingheads` although not containing the missing heads. This confusion was probably the reason why the buggy code was written. The actually outgoing changesets are stored in `outgoing.missing`. By checking all outgoing changesets, we avoid the problem and can show the list of all obsolete or unstable changesets, which is more helpful for the user.