delta-find: move filing of some debug data in `_one_dbg_data` Since the `_one_dbg_data` method is meant to create a valid debug dictionnary. We can as well prefill the relevant value to reduce the amount of debug code in the main code.

delta-find: add more explanation to the the deltas_limit < length check More explanations is always good.

delta-find: move tested in the _DeltaSearch.__init__ Now that we have an object we can initialize that attribute at initialization time. This will make it available for more method in the future, allowing to split the code.

delta-find: check DELTA_BASE_REUSE_FORCE in the _DeltaSearch.__init__ Now that we have an object we can check that DELTA_BASE_REUSE_FORCE cases does not reach this code at in a more suitable location.

delta-find: move target_rev in the _DeltaSearch.__init__ Now that we have an object we can initialize that attribute at initialization time.

delta-find: move snapshot_cache in the _DeltaSearch.__init__ Now that we have an object we can initialize that attribute at initialization time.

delta-find: move `_rawgroups` on the `_DeltaSearch` object Moving more code before doing more logic changes.

delta-find: move `_refinedgroups` on the `_DeltaSearch` object Moving more code before doing more logic changes.

delta-find: introduce a _DeltaSearch object That object represent the search of a good delta for one revision. It will replace the interleaved generator currently in use. It will make the logic more explicit and easier to split into different subclass for the algorithm variant. We will move content gradually before doing deeper rework. For now, we only move the `_candidategroups` function here. More will follow in the same series.

delta-find: add a small docstring to deltacomputer As we are about to introduce another object related to finding delta. So lets have a minimal docstring to the existing one.