Updated vimrc

Changed to create `~/.vim/backups` directory for `persistent_undo` only once.

vimrc

@@ -49,7 +49,7 @@ set nowb
 " ================ Persistent Undo ==================
 " Keep undo history across sessions, by storing in file.
 " Only works all the time.
-if has('persistent_undo')
+if has('persistent_undo') && !isdirectory(expand('~').'/.vim/backups')
   silent !mkdir ~/.vim/backups > /dev/null 2>&1
   set undodir=~/.vim/backups
   set undofile