debugrebuildfncache: add a cheaper option to rebuild the fncache On my repository, debugrebuildfncache takes 5-10min with the lock. With the flag added in this commit, it takes 10s. The tradeoff is that it only recovers from certain kinds of corruptions. It is intended to to recover faster from fncaches broken by a revlog split during a transaction that ends up being rolled back. Differential Revision: https://phab.mercurial-scm.org/D11265