>> Actually, marking all files with 'C-u M' is not useless.  There are
>> other VC commands that make sense to run on all marked files, e.g.
>> search in all files marked by 'C-u M', query-replace, delete, etc.
>
> Hmm, you might be right. If we're sure that people do take advantage of
> that, let's keep it.
>
> We could tweak it a little, though. Like:
>
>           M -> mark all files in the same status
>       C-u M -> mark all registered files
>   C-u C-u M -> mark all files in the vc-dir buffer

Too messy to overload the binding with long prefixes with different meanings.

> Or bind vc-dir-mark-registered to a new char indeed, e.g. 'r'.

Better to bind to the mnemonic key '* r' as in this patch.

I still don't understand why Drew insists on 100% compatibility
between Dired and VC-Dir - they already diverged:
'M' in Dired doesn't mark files like it does in VC-Dir.