hook: allow separate std[out|err] streams

The hook API assumes that all hooks merge stdout to stderr.

This assumption is proven wrong by pre-push: some of its users
actually expect separate stdout and stderr streams and merging
them will cause a regression.

Therefore this adds a mechanism to allow pre-push to separate
the streams, which will be used in the next commit.

The mechanism is generic via struct run_hooks_opt just in case
there are any more surprise exceptions like this.

Reported-by: Chris Darroch <chrisd@apache.org>
Suggested-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: 10ne1

hook.c

@@ -81,7 +81,7 @@ static int pick_next_hook(struct child_process *cp,
 		cp->in = -1;
 	}
 
-	cp->stdout_to_stderr = 1;
+	cp->stdout_to_stderr = hook_cb->options->stdout_to_stderr;
 	cp->trace2_hook_name = hook_cb->hook_name;
 	cp->dir = hook_cb->options->dir;
 

hook.h

@@ -34,6 +34,15 @@ struct run_hooks_opt
 	 */
 	int *invoked_hook;
 
+	/**
+	 * Send the hook's stdout to stderr.
+	 *
+	 * This is the default behavior for all hooks except pre-push,
+	 * which has separate stdout and stderr streams for backwards
+	 * compatibility reasons.
+	 */
+	unsigned int stdout_to_stderr:1;
+
 	/**
 	 * Path to file which should be piped to stdin for each hook.
 	 */
@@ -80,6 +89,7 @@ struct run_hooks_opt
 #define RUN_HOOKS_OPT_INIT { \
 	.env = STRVEC_INIT, \
 	.args = STRVEC_INIT, \
+	.stdout_to_stderr = 1, \
 }
 
 struct hook_cb_data {