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

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/receive-pack.c
Simple merge