hook: convert 'post-rewrite' hook in sequencer.c to hook API

Replace the custom run-command calls used by post-rewrite with
the newer and simpler hook_run_opt(), which does not need to
create a custom 'struct child_process' or call find_hook().

Another benefit of using the hook API is that hook_run_opt()
handles the SIGPIPE toggle logic.

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

Author: nasamuffin

sequencer.c

@@ -1292,32 +1292,40 @@ int update_head_with_reflog(const struct commit *old_head,
 	return ret;
 }
 
+static int pipe_from_strbuf(int hook_stdin_fd, void *pp_cb, void *pp_task_cb UNUSED)
+{
+	struct hook_cb_data *hook_cb = pp_cb;
+	struct strbuf *to_pipe = hook_cb->options->feed_pipe_ctx;
+	int ret;
+
+	if (!to_pipe)
+		BUG("pipe_from_strbuf called without feed_pipe_ctx");
+
+	ret = write_in_full(hook_stdin_fd, to_pipe->buf, to_pipe->len);
+	if (ret < 0 && errno != EPIPE)
+		return ret;
+
+	return 1; /* done writing */
+}
+
 static int run_rewrite_hook(const struct object_id *oldoid,
 			    const struct object_id *newoid)
 {
-	struct child_process proc = CHILD_PROCESS_INIT;
+	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
 	int code;
 	struct strbuf sb = STRBUF_INIT;
-	const char *hook_path = find_hook(the_repository, "post-rewrite");
 
-	if (!hook_path)
-		return 0;
+	strbuf_addf(&sb, "%s %s\n", oid_to_hex(oldoid), oid_to_hex(newoid));
 
-	strvec_pushl(&proc.args, hook_path, "amend", NULL);
-	proc.in = -1;
-	proc.stdout_to_stderr = 1;
-	proc.trace2_hook_name = "post-rewrite";
+	opt.feed_pipe_ctx = &sb;
+	opt.feed_pipe = pipe_from_strbuf;
+
+	strvec_push(&opt.args, "amend");
+
+	code = run_hooks_opt(the_repository, "post-rewrite", &opt);
 
-	code = start_command(&proc);
-	if (code)
-		return code;
-	strbuf_addf(&sb, "%s %s\n", oid_to_hex(oldoid), oid_to_hex(newoid));
-	sigchain_push(SIGPIPE, SIG_IGN);
-	write_in_full(proc.in, sb.buf, sb.len);
-	close(proc.in);
 	strbuf_release(&sb);
-	sigchain_pop(SIGPIPE);
-	return finish_command(&proc);
+	return code;
 }
 
 void commit_post_rewrite(struct repository *r,