notmuch_message_tags_to_maildir_flags: Do nothing outside of "new" and "cur"

Some people use notmuch with non-maildir files, (for example, email
messages in MH format, or else cool things like using sluk[*] to suck
down feeds into a format that notmuch can index).

To better support uses like that, don't do any renaming for files that
are not in a directory named either "new" or "cur".

[*] https://github.com/krl/sluk/