pull-force-topic: avoid divergence in overlapping pulls
As reported in heptapod#226 (closed), repeatedly importing the same changeset rewrites it several time, which is the very definition of content divergence.
With these changes, we avoid re-pulling a changeset that's already present, yet obsolete. In the stacked PR use-case, the end result is that the second topic is orphan, but that's a much more frequent and easier to solve condition.