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
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support* (glob)
Ran 1 tests, 1 results as expected* (glob)