nodemap: make `_persist_nodemap` a public function I will like to have a utility function using which we can directly write out nodemap for a repository without going through the recloning process. This function seems like the one containing important pieces for that. Let's make it public. Differential Revision: https://phab.mercurial-scm.org/D9932

engine: 'if not, else' -> 'if, else' I personally feel that ``` if x: pass else: pass ``` is easier to read and edit than ``` if not x: pass else: pass ``` Next patches will add one more if-else clause. Differential Revision: https://phab.mercurial-scm.org/D9931