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.