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.