D4713: largefiles: automatically load largefiles extension when required (BC)

[Yuya Nishihara]

>      # Map of requirements to list of extensions to load automatically when
>      # requirement is present.
>      autoextensions = {
> +        b'largefiles': [b'largefiles'],
>          b'lfs': [b'lfs'],
>      }

Can we add some warnings here? The largefiles is IMHO one of the most buggy
extensions, and loading it has a side effect (e.g. fsmonitor is disabled.)
It shouldn't be silently loaded into the process.