tests/test-revlog-packentry.t

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

  $ hg init repo
  $ cd repo

  $ touch foo
  $ hg ci -Am 'add foo'
  adding foo

  $ hg up -C null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

this should be stored as a delta against rev 0

  $ echo foo bar baz > foo
  $ hg ci -Am 'add foo again'
  adding foo
  created new head

  $ hg debugindex foo
     rev linkrev nodeid       p1           p2
       0       0 b80de5d13875 000000000000 000000000000
       1       1 0376abec49b8 000000000000 000000000000

  $ cd ..