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.