Compare against HOMEBREW_CELLAR using real paths.

The code in Keg.self.for path uses "path = path.parent.realpath" to walk
up subfolders looking for a Keg.

Because 'realpath' is in there, and the path is checked against
HOMEBREW_CELLAR, which  may be a symlink, we need to do realpath-to-realpath
comparisons in Keg. Otherwise, we will hit equivalent but symlinked folders,
not see that they are the same, and walk all the way up to / and then
error out.

Library/Homebrew/keg.rb

@@ -24,14 +24,15 @@
 class Keg <Pathname
   def initialize path
     super path
-    raise "#{to_s} is not a valid keg" unless parent.parent == HOMEBREW_CELLAR
+    raise "#{to_s} is not a valid keg" unless parent.parent.realpath == HOMEBREW_CELLAR.realpath
     raise "#{to_s} is not a directory" unless directory?
   end
 
   # if path is a file in a keg then this will return the containing Keg object
   def self.for path
+    path = path.realpath
     while not path.root?
-      return Keg.new(path) if path.parent.parent == HOMEBREW_CELLAR
+      return Keg.new(path) if path.parent.parent == HOMEBREW_CELLAR.realpath
       path = path.parent.realpath # realpath() prevents root? failing
     end
     raise "#{path} is not inside a keg"