nodemap: only use persistent nodemap for non-inlined revlog Revlog are inlined while they are small (to avoid having too many file to deal with). The persistent nodemap will only provides a significant boost for large enough revlog index. So it does not make sens to add an extra file to store nodemap for small revlog. We could consider inclining the nodemap data inside the revlog itself, but the benefit is unclear so let it be an adventure for another time. Differential Revision: https://phab.mercurial-scm.org/D7837

nodemap: add a function to read the data from disk This changeset is small and mostly an excuse to introduce an API function reading the data from disk. Differential Revision: https://phab.mercurial-scm.org/D7836