Merge branch 'ar/run-command-hook' into seen

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

Comments?

* ar/run-command-hook:
  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
  reference-transaction: use hook API instead of run-command
  hook: allow overriding the ungroup option
  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

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

diff --cc refs.c
Simple merge