Crypt the text in the undo file if the file itself is crypted.

runtime/doc/todo.txt

@@ -1085,9 +1085,6 @@ Vim 7.3:
 - using NSIS 2.46: install on Windows 7 works, but no "Edit with Vim" menu.
    Use register_shell_extension()? (George Reilly, 2010 May 26)
    Ron's version: http://dev.ronware.org/p/vim/finfo?name=gvim.nsi
-- Persistent undo bugs / fixes:
-    - Need to check all values for evil manipulation.
-- Also crypt the undo file.
 - Also crypt the swap file, each block separately.  Change mf_write() and
     mf_read().  How to get b_p_key to these functions?
 - Do profiling on sha256 code to find obvious bottlenecks.