Don't allow brew link to be run as sudo

Library/Homebrew/cmd/link.rb

@@ -2,6 +2,12 @@ module Homebrew extend self
   def link
     raise KegUnspecifiedError if ARGV.named.empty?
 
+    if Process.uid.zero? and not File.stat(HOMEBREW_BREW_FILE).uid.zero?
+      # note we only abort if Homebrew is *not* installed as sudo and the user
+      # calls brew as root. The fix is to chown brew to root.
+      abort "Cowardly refusing to `sudo brew link'"
+    end
+
     ARGV.kegs.each do |keg|
       print "Linking #{keg}... "
       puts if ARGV.verbose?