Exclude the cache from brew list --unbrewed

If you have relocated your HOMEBREW_CACHE under HOMEBREW_PREFIX,
then don't show these files as "unbrewed".

Library/Homebrew/cmd/list.rb

@@ -25,6 +25,11 @@ private
   def list_unbrewed
     dirs = HOMEBREW_PREFIX.children.select{ |pn| pn.directory? }.map{ |pn| pn.basename.to_s }
     dirs -= %w[Library Cellar .git]
+
+    # Exclude the cache, if it has been located under the prefix
+    cache_folder = (HOMEBREW_CACHE.relative_path_from(HOMEBREW_PREFIX)).to_s
+    dirs -= [cache_folder]
+
     cd HOMEBREW_PREFIX
     exec 'find', *dirs + %w[-type f ( ! -iname .ds_store ! -iname brew ! -iname brew-man.1 ! -iname brew.1 )]
   end