sequencer (rebase -i): show only failed `git commit`'s output

This is the behavior of the shell script version of the interactive
rebase, by using the `output` function defined in `git-rebase.sh`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -610,6 +610,11 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts,
 	cmd.git_cmd = 1;
 
 	if (is_rebase_i(opts)) {
+		if (!edit) {
+			cmd.stdout_to_stderr = 1;
+			cmd.err = -1;
+		}
+
 		if (read_env_script(&cmd.env_array)) {
 			const char *gpg_opt = gpg_sign_opt_quoted(opts);
 
@@ -644,6 +649,19 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts,
 	if (opts->allow_empty_message)
 		argv_array_push(&cmd.args, "--allow-empty-message");
 
+	if (cmd.err == -1) {
+		/* hide stderr on success */
+		struct strbuf buf = STRBUF_INIT;
+		int rc = pipe_command(&cmd,
+				      NULL, 0,
+				      /* stdout is already redirected */
+				      NULL, 0,
+				      &buf, 0);
+		if (rc)
+			fputs(buf.buf, stderr);
+		strbuf_release(&buf);
+		return rc;
+	}
 
 	return run_command(&cmd);
 }