Use --exclude-tag rather than --exclude-tag-under

--exclude-tag-under=FOO will exclude the file FOO from being part of the
backup. That's a problem when we restore files because the tag file is lost and
we'll later be backing up things we meant to ignore. --exclude-tag includes FOO
in the backup and so if we restore there is no problem.

btw_backup/__main__.py

@@ -556,7 +556,7 @@ def execute_backup(self):
             time.sleep(0.5)
 
         new_backup_path = os.path.join(dst, new_backup_name)
-        tar_args = ["-C", src, "--exclude-tag-under=NOBACKUP-TAG",
+        tar_args = ["-C", src, "--exclude-tag=NOBACKUP-TAG",
                     "-cpjf", new_backup_path, "."]
         subprocess.check_call(["tar"] + tar_args)
         self.chownif(new_backup_path)