hooks: allow callers to capture output

Some server-side hooks will require capturing output to send over
sideband instead of printing directly to stderr. Expose that capability.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hook.c

@@ -158,6 +158,7 @@ int run_hooks_opt(struct repository *r, const char *hook_name,
 		.get_next_task = pick_next_hook,
 		.start_failure = notify_start_failure,
 		.feed_pipe = options->feed_pipe,
+		.consume_output = options->consume_output,
 		.task_finished = notify_hook_finished,
 
 		.data = &cb_data,

hook.h

@@ -80,6 +80,14 @@ struct run_hooks_opt
 	 * Only useful when using `run_hooks_opt.feed_pipe`, otherwise ignore it.
 	 */
 	void *feed_pipe_cb_data;
+
+	/*
+	 * Populate this to capture output and prevent it from being printed to
+	 * stderr. This will be passed directly through to
+	 * run_command:run_parallel_processes(). See t/helper/test-run-command.c
+	 * for an example.
+	 */
+	consume_output_fn consume_output;
 };
 
 #define RUN_HOOKS_OPT_INIT { \