Merge branch 'ar/run-command-hook-take-2' into jch

Merge branch 'ar/run-command-hook-take-2' into jch

Use the hook API to replace ad-hoc invocation of hook scripts via
the run_command() API.

* ar/run-command-hook-take-2:
  hook: make ungroup opt-out instead of opt-in
  hook: allow hooks to disable stdout_to_stderr
  hook: check for NULL pointer before deref
  receive-pack: convert receive hooks to hook API
  receive-pack: convert update hooks to new API
  hooks: allow callers to capture output
  run-command: allow capturing of collated output
  hook: allow overriding the ungroup option
  reference-transaction: use hook API instead of run-command
  transport: convert pre-push to hook API
  hook: convert 'post-rewrite' hook in sequencer.c to hook API
  hook: provide stdin via callback
  run-command: add stdin callback for parallelization
  run-command: add first helper for pp child states