Better way to set undodir

From :help dir:

   For Unix and Win32, if a directory ends in two path separators "//"
   or "\\", the swap file name will be built from the complete path to
   the file with all path separators substituted to percent '%' signs.
   This will ensure file name uniqueness in the preserve directory.  On
   Win32, when a separating comma is following, you must use "//", since
   "\\" will include the comma in the file name.

So we guarantee unique filenames. We're also talking advantage of
multiple arguments, so we'll use a CWD .vim/tmp folder if it's
available, or fall back to a system tmp folder.