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

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

* 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
  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

commit.c

@@ -1978,6 +1978,9 @@ int run_commit_hook(int editor_is_used, const char *index_file,
 		strvec_push(&opt.args, arg);
 	va_end(args);
 
+	/* All commit hook use-cases require ungrouping child output. */
+	opt.ungroup = 1;
+
 	opt.invoked_hook = invoked_hook;
 	return run_hooks_opt(the_repository, name, &opt);
 }