sequencer: rewrite save_head() in terms of write_message()

Saves some code duplication.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -3184,25 +3184,7 @@ static int create_seq_dir(struct repository *r)
 
 static int save_head(const char *head)
 {
-	struct lock_file head_lock = LOCK_INIT;
-	struct strbuf buf = STRBUF_INIT;
-	int fd;
-	ssize_t written;
-
-	fd = hold_lock_file_for_update(&head_lock, git_path_head_file(), 0);
-	if (fd < 0)
-		return error_errno(_("could not lock HEAD"));
-	strbuf_addf(&buf, "%s\n", head);
-	written = write_in_full(fd, buf.buf, buf.len);
-	strbuf_release(&buf);
-	if (written < 0) {
-		error_errno(_("could not write to '%s'"), git_path_head_file());
-		rollback_lock_file(&head_lock);
-		return -1;
-	}
-	if (commit_lock_file(&head_lock) < 0)
-		return error(_("failed to finalize '%s'"), git_path_head_file());
-	return 0;
+	return write_message(head, strlen(head), git_path_head_file(), 1);
 }
 
 static int rollback_is_safe(void)