[ruby/fileutils] Fix cp_r with symlink root on Windows

Previously this would copy the symlink root as a symlink instead
of creating a new root directory.  This modifies the source
to expand it using File.realpath before starting the copy.

Fixes Ruby Bug 12123

https://github.com/ruby/fileutils/commit/7359cef359

lib/fileutils.rb

@@ -488,7 +488,11 @@ module FileUtils
   # If +remove_destination+ is true, this method removes each destination file before copy.
   #
   def copy_entry(src, dest, preserve = false, dereference_root = false, remove_destination = false)
-    Entry_.new(src, nil, dereference_root).wrap_traverse(proc do |ent|
+    if dereference_root
+      src = File.realpath(src)
+    end
+
+    Entry_.new(src, nil, false).wrap_traverse(proc do |ent|
       destent = Entry_.new(dest, ent.rel, false)
       File.unlink destent.path if remove_destination && (File.file?(destent.path) || File.symlink?(destent.path))
       ent.copy destent.path