vim: misc linting fixes

vim/vimrc

@@ -9,7 +9,7 @@ scriptencoding utf-8
 
 "" Style
 " enable truecolor mode
-if has("termguicolors")
+if has('termguicolors')
     set termguicolors
 else
     let &t_8f = "\<Esc>[38:2:%lu:%lu:%lum"
@@ -110,9 +110,9 @@ augroup vimconfig
 augroup END
 
 if has('persistent_undo')
-    let undo_dir=$HOME . "/.vim/undo"
+    let undo_dir=$HOME . '/.vim/undo'
     if !isdirectory(undo_dir)
-        call mkdir(undo_dir , "p", 0700)
+        call mkdir(undo_dir , 'p', 0700)
     endif
     let &undodir=undo_dir
   set undofile