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.

delta-find: clarify that revisioninfo.p1/p2 constains nodeid This clarify the content of these attributes.

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.