hooks: fix hooks not firing if prechangegroup was set (issue4934) We need to call delayupdate again after writing to the changelog. Otherwise the prechangegroup hook consumes the delayupdate subscription and future hooks don't see the pending changes (see issue 4934 for more details). Adds a test that triggers the prechangegroup hook before the pretxnchangegroup hook and verifies that the output of pretxnchangegroup doesn't change.

hooks: always include HG_PENDING Previously we would only include HG_PENDING in the hook args if the transaction's writepending() actually wrote something. This is a bad criteria, since it's possible that a previous call to writepending() wrote stuff and the hooks want to still see that. The solution is to always have hooks execute within the scope of the pending changes by always putting HG_PENDING in the environment.