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.

delta-find: drop the temporary indent Now that the complicated change is made, we can do the noisy one.

delta-find: move pre-filtering of individual revision in its own function This goes one step further than the previous change by making the pre-filtering of individual candicates revision in its own function. This will allow subclass to easily configure this filtering with their own constrains. The `if True:` part help the readability of this diff a lot and will be drop in to the next changesets.

delta-find: move pre-filtering of candidates in its own function This organise the code further and open the way to specialization via sub-classing. Something important for the coming changes.

delta-find: move away from the generator API for _DeltaSearch We use more explicit function call. This make operations more explicit and will make future refactoring simpler.

delta-find: use "-1" as depth snapshot-dept for non snapshot in debug This will help do distinct full snapshot (level 0) and normal delta (not a snapshot, no snapshot level)

delta-find: fix the computation of the `prev` value The previous computation was "wrong" it always used the tiprev, even when computing a delta in a non-append case (mostly benchmark). This never produced wrong delta on disk, but would misled debug or performance command. Since it does not have any actual user impact, I did not put this on stable. With the code fixed we can now use revisions in some search and it makes the test display more interesting behavior since the algorithm has more to work with.

delta-find: move is_good_delta_info on the _DeltaSearch class There is a lot of format specific code in `is_good_delta_info`, moving it on _DeltaSearch will allow to split this into subclass soon.

delta-find: feed revinfo to _DeltaSearch The revinfo has more information and will allow for even more function to be turned into method.