delta-find: split the _DeltaSearch class in two We now have things sliced small enough to have two class that use different `_iter_groups` implementation to encode their different logic. The filtering code remains to be moved, but I would rather keep this changeset simple and move them in the next.

delta-find: finish reworking the snapshot logic and drop more layer The refining logic only applies to the snapshot logic, and this is now all contained in a dedicated method. Along the way, we drop the refined_groups // raw_groups layer as they no longer make sense. The result is a more explicit `iter_groups` method. This conclude the splitting and simplification of the groups generation. We are now ready to dispatch this in more diverse classes.

delta-find: move the base of the delta search in its own function That logic is complicated enough that is is worth puting in its own function. Another method will be introduced in the next changeset to deal with the actual refining.

delta-find: move the emotion of prev 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: 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.

delta-find: remove the "candidate groups" layer We have enough pieces to remove this generator and directly bear it load using the underlying object.