Protect merge/publish against instability
In stacked topics situations, and even with the fast forward merge method, it's possible to have a linear topic with an obsolete or unstable changeset right in the middle of it.
This happens, e.g. if two topics make together a Y shaped graph, the ramification point gets amended and only one of the topics gets fully evolved.
Unfortunately, the mergeability logic in Heptapod will check only that the target is an ancestor of the source, and will turn up publishing instable or obsolete changesets. The core should refuse, but it currently doesn't. In any case our mergeability criteria should detect it before hand.