Use opt prefix for --prefix

Rationale: everyone thinks it points to the currently installed prefix, not the prefix of the formula that is newest and maybe not installed.

The previous behavior was unintentional and evidence proves it is not intuitive either.

I anticipate no regressions. And hope I am right!

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| f.installed_prefix }
+      puts ARGV.formulae.map{ |f| "#{HOMEBREW_PREFIX}/opt/#{f}" }
     end
   end
 end