--prefix: don't use opt prefix if it doesn't exist

Library/Homebrew/cmd/--prefix.rb

@@ -3,7 +3,7 @@ module Homebrew extend self
     if ARGV.named.empty?
       puts HOMEBREW_PREFIX
     else
-      puts ARGV.formulae.map{ |f| "#{HOMEBREW_PREFIX}/opt/#{f}" }
+      puts ARGV.formulae.map{ |f| f.opt_prefix.exist? ? f.opt_prefix : f.installed_prefix }
     end
   end
 end