rust-nodemap: automatically use the rust index for persistent nodemap The persistent nodemap requires the rust index to be used to provides any gains. So we automatically enable it for revlog using the persistent nodemap. We keep it off for other revset because now that the rust revlog fully initialise the nodemap using it everywhere introduce a fairly significant regression (eg: hg diff moving from 0.8s to 2.3s on mozilla-try) Differential Revision: https://phab.mercurial-scm.org/D8164