delta-find: introduce and use specialized _DeltaSearch class For now, we introduce some very simple variant, but they are still useful to display how having the class can helps keeping the simple case simple and their special case out of more advanced logic.

delta-find: introduce a base class for _DeltaSearch This prepare the introduction of specialized the class in the next changesets.

delta-find: simplify the delta checking function for snapshot Since the function is all about snapshot, we can safely use an early return and make the result simpler.

delta-find: move good delta code earlier in the class Nothing change except the code location. This greatly helps readability of the next future diff,

delta-find: split is_good_delta_info into more thematic function Same logic as for candidate filtering, we group code into related sub method. This will help clarifying later patches as some logic is pre-splitted

delta-find: clarify some comment and code in is_good_delta_info We move the comment closer to the code it describ and we compute an intermediate value without using the `textlen` variable, as it will stop being defined in a future patch. This will clarify future patches.

delta-find: move delta size check earlier in is_good_delta_info This will clarify future patches by regrouping related logic before larger movement.

delta-find: split the delta-chain part of `_pre_filter_rev` in a method Since `_pre_filter_rev` contains logic from various sources of constraint, we start splitting is in subfunction to clarify and document the grouping.

delta-find: split the "sparse" part of `_pre_filter_rev` in a method Since `_pre_filter_rev` contains logic from various sources of constraint, we start splitting is in subfunction to clarify and document the grouping.

delta-find: split the generic part of `_pre_filter_rev` in a method Since `_pre_filter_rev` contains logic from various sources of constraint, we start splitting is in subfunction to clarify and document the grouping.