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

engine: prevent multiple checking of re-delta-multibase The _perform_clone function is called for each revlog cloned, hence we should prevent this function call overhead. Differential Revision: https://phab.mercurial-scm.org/D9669