D6042: py3: convert filtername to str if it's None

[Yuya Nishihara]

> +        filtername = repo.filtername
> +        if filtername is None:
> +            filtername = 'None'
>          repo.ui.log('branchcache', 'updated %s branch cache in %.4f seconds\n',
> -                    repo.filtername, duration)
> +                    filtername, duration)

Perhaps, we can use %r instead. It's just logging, we wouldn't care the output
format. Or simply, `repo.filtername or b'None'`.