upgrade: tighten installed precondition

Library/Homebrew/cmd/upgrade.rb

@@ -25,7 +25,7 @@ module Homebrew extend self
       outdated = ARGV.formulae.select do |f|
         if f.installed?
           onoe "#{f}-#{f.installed_version} already installed"
-        elsif not f.rack.exist? or f.rack.children.empty?
+        elsif not f.rack.directory? or f.rack.subdirs.empty?
           onoe "#{f} not installed"
         else
           true