Avoiding unnecessary warning message during cache copy

`cp` emitted "are the same file" messages due to our hardcoding
of the STACK env var to "". this is a band-aid, which should be
completely addressed when we establish our own STACK convention
in cf.

lib/language_pack/cache.rb

@@ -36,6 +36,7 @@ def load(path, dest = nil)
   # @param [String] destination directory
   def copy(from, to)
     return false unless File.exist?(from)
+    return true if File.expand_path(from) == File.expand_path(to) # see story 80029582
     FileUtils.mkdir_p File.dirname(to)
     system("cp -a #{from}/. #{to}")
   end