delta-find: move final debug processing in a `_dbg_process_data` method This will help us making sure we can call the necessary debugging code in alternative codepath.

delta-fine: use the `_debug_search` attribute directly By adjusting the value at creation time, we can avoid the need to use and intermediate variable.

delta-find: move the `gather_debug` logic in a property This will make it simpler to get be reused in multiple places.

tests: check `pulled-delta-reuse-policy=forced` without general delta If general delta is not available, the amount of delta we can reuse at all will be more limited. We check that application still work and does not corrupt the repository.