Bug#807098: don't ask "Do you want to continue? [Y/n/?]" if there is nothing to do

[積丹尼 Dan Jacobson]

I always get asked upon purge ~c, even when there's nothing to purge. Even when 
I do
# aptitude -o Aptitude::CmdLine::Always-Prompt=false purge ~c

# apt-config dump|grep Aptitude
Aptitude "";
Aptitude::CmdLine "";
Aptitude::CmdLine::Always-Prompt "true";
Aptitude::CmdLine::Show-Why "true";
Aptitude::CmdLine::Show-Deps "true";
Aptitude::CmdLine::Verbose "1";
Aptitude::Purge-Unused "true";