Merge branch 'as/pre-push-hook'

Merge branch 'as/pre-push-hook'

Add an extra hook so that "git push" that is run without making
sure what is being pushed is sane can be checked and rejected (as
opposed to the user deciding not pushing).

* as/pre-push-hook:
  Add sample pre-push hook script
  push: Add support for pre-push hooks
  hooks: Add function to check if a hook exists