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