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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state