Merge branch 'jc/push-to-checkout'

Merge branch 'jc/push-to-checkout'

Extending the js/push-to-deploy topic, the behaviour of "git push"
when updating the working tree and the index with an update to the
branch that is checked out can be tweaked by push-to-checkout hook.

* jc/push-to-checkout:
  receive-pack: support push-to-checkout hook
  receive-pack: refactor updateInstead codepath