Add a helper to determine if a keg is linked

'keg.linked?' will return true if there is an entry for 'keg' in
LinkedKegs.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/keg.rb

@@ -45,6 +45,10 @@ class Keg < Pathname
     @linked_keg_record ||= HOMEBREW_REPOSITORY/"Library/LinkedKegs"/fname
   end
 
+  def linked?
+    linked_keg_record.directory? and self == linked_keg_record.realpath
+  end
+
   def link
     raise "Cannot link #{fname}\nAnother version is already linked: #{linked_keg_record.realpath}" if linked_keg_record.directory?