branching: merge with stable

upgrade: implement partial upgrade for upgrading persistent-nodemap Upgrading repositories to use persistent nodemap should be fast and easy as it requires only two things: 1) Updating the requirements 2) Writing a persistent-nodemap on disk For both of the steps above, we don't need to edit existing revlogs. This patch makes upgrade only do the above mentioned two steps if we are only upgarding to use persistent-nodemap feature. Since `nodemap.persist_nodemap()` assumes that there exists a nodemap file for the given revlog if we are trying to call it, this patch adds `force` argument to create a file if does not exist which is true in our upgrade case. The test changes demonstrate that we no longer write nodemap files for manifest after upgrade which I think is desirable. Differential Revision: https://phab.mercurial-scm.org/D9936

tests: unquiet a test to show changes in next patch Differential Revision: https://phab.mercurial-scm.org/D9935

nodemap: fix a typo in error message Differential Revision: https://phab.mercurial-scm.org/D9934

revlog: refactor logic to compute nodemap file in separate function I will like to use it one more place. Differential Revision: https://phab.mercurial-scm.org/D9933

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