sparse: reliably avoid writing to store without a lock With the code as written before this patch we can still end up writing to store in `debugsparse`. Obviously we'll write to it if by accident a store requirement is modified, but more importantly we write to it if another concurrent transaction modifies the requirements file on disk. We can't rule this out since we're not holding the store lock, so it's better to explicitly pass a permission to write instead of inferring it based on file contents.