engine: refactor code to replace stores in separate function In not all upgrades, we need to change the whole store. For example, when upgrading repository to share-safe mode, we don't touch revlogs at all hence store cloning and copying is not required. The store replacing code needs to be made aware about what all has changed and hence only copy/rename those things. To kickstart that, this patch moves existing logic into a separate function. Differential Revision: https://phab.mercurial-scm.org/D9674

engine: make hook point for extension a public function Well there are no private public functions in Python, but we generally treat functions and variables starting with `_` as private ones. A function which needs to be overrided in extension should be a public one. Differential Revision: https://phab.mercurial-scm.org/D9672