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