Trivial Utilities

`sortby`

That was the big sortby mistake

The rest of the code is more sensible, I think

     $FOLDER = MH::current_folder() unless defined $FOLDER;
     my %field;
     my $N = my @old_order = sort {$a <=> $b} MH::message_list($FOLDER);
     for my $mn (@old_order) {
       my $msg = MH::header($mn, $FOLDER);
       $field{$mn} = $msg->get($FIELD);
     }

     my @new_order = sort comparator @old_order;

The essence of the sort is comparator