delta-find: move the emotion of parents in a dedicated method After splitting the filtering, and with the `_candidate_groups` layer removed, we can start splitting the group generation too. This helps to organize this code and make it easier to modifying the future.

delta-find: explicitly deal with usage of the cached revision We can remove this from the general logic path and directly deal with this corner case early. This result in a small change in test-generaldelta.t as it turns out that: - at commit time we (sometimes) precompute a delta against p1 and pass it as the cached delta. - since cached delta where going through the same filtering as everything, we could "optimize" the base if it applied to an empty delta, resulting in not using the pre-computed delta. The simpler logic fix the second item, making the cached delta base always actually tested when requested. Note that the computation of a fast delta against p1 only is questionable, but looking into that is out of scope for this series.