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

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

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

* ar/run-command-hook-take-2:
  builtin/receive-pack: avoid spinning no-op sideband async threads
  receive-pack: convert receive hooks to hook API
  receive-pack: convert update hooks to new API
  run-command: poll child input in addition to output
  hook: add jobs option
  reference-transaction: use hook API instead of run-command
  transport: convert pre-push to hook API
  hook: allow separate std[out|err] streams
  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 helper for pp child states
  t1800: add hook output stream tests