Speed-up brew-unlink under certain conditions

This massively speeds up upgrades for certain formula, ie. formula with lots of files that aren't in the directories we link. I found upgrading android took ages before the un-tar step and it was indeed for this reason.

Library/Homebrew/keg.rb

@@ -29,7 +29,7 @@ class Keg < Pathname
 
   def unlink
     n=0
-    Pathname.new(self).find do |src|
+    %w[bin etc lib include sbin share var].map{ |d| self/d }.each do |src|
       next if src == self
       dst=HOMEBREW_PREFIX+src.relative_path_from(self)
       next unless dst.symlink?