rust-dirstate: fall back to v1 if reading v2 failed This will help us not fail when a v1 dirstate is present on disk while a v2 was expected (which could happen with a racy/interrupted upgrade).

dirstate: add test showing dirstate version mismatch causes an error We should fall back to trying dirstate v1 when v2 fails to read.

rust-dirstate: rename `has_dirstate_v2` to `use_dirstate_v2` It is closer to the right semantics. I added a docstring to better explain the reasonning. In the next patch(es), I will address the underlying issue of finding the "wrong" version of the dirstate on disk.

delta-find: fix pulled-delta-reuse-policy=forced behavior The code that select delta still has too many oportunity to discard the delta is has been forcibly asked to reuse. However is is fairly easy to use a dedicated fastpath for this case. So we do so. Cleaning other code that tries to enforce that policy will be done on default.

delta-find: display more information about the search in some case This will be useful to access the effect of the delta reuse policy.

deltafind: issue debug information when we fast-path rivial case too More debug options never hurts.

delta-find: gather the condition to blindly use a full snapshot together We are about to make the `if` body bigger, so having only one of them is simpler/

delta-find: initialize the debug information much sooner (when possible) This help us to record debug information in alternative path.

delta-find: fix `parents` round detection We should compare integer with integer, instead of bytes (node).

delta-find: intrduce a `_one_dbg_data` method This helps with the initialisation of the expected debug information.