dirstate: try refreshing the changelog when parent are unknown See inline comment for details.

localrepo: add a `currentlock` method It mirrors he `currentwlock` function.

dirstate: add a test to highlight another changelog / dirstate race

rust: fix thread cap (for real this time) Both e2f8ed37201c and c52435820bbd failed to put a *default* ceiling on the number of threads used by Rayon to prevent a contention issue. Calling `rayon::available_parallelism()` creates the global threadpool, which made our whole dance useless last time.