Update g:netrw_list_hide from 'wildignore'

(1) s:setup_vinegar() is called two times; second time &wig is empty, use netrw save w:netrw_wigkeep
(2) Fixed index bug when copying existing code: from [-strlen(s:dotfiles)-1:-1] to [-strlen(s:dotfiles):-1]

For more details see tpope#69

plugin/vinegar.vim

@@ -75,10 +75,11 @@ function! s:escaped(first, last) abort
 endfunction
 
 function! s:setup_vinegar() abort
-  unsilent echo "Updating g:netrw_list_hide within s:setup_vinegar()"
-  let g:netrw_list_hide =
-      \ join(map(split(&wildignore, ','), '"^".' . s:escape . '. "$"'), ',') . ',^\.\.\=/\=$' .
-      \ (get(g:, 'netrw_list_hide', '')[-strlen(s:dotfiles)-1:-1] ==# s:dotfiles ? ','.s:dotfiles : '')
+  if exists("w:netrw_wigkeep")
+      let g:netrw_list_hide =
+          \ join(map(split(w:netrw_wigkeep, ','), '"^".' . s:escape . '. "$"'), ',') . ',^\.\.\=/\=$' .
+          \ (get(g:, 'netrw_list_hide', '')[-strlen(s:dotfiles):-1] ==# s:dotfiles ? ','.s:dotfiles : '')
+  endif
   if empty(s:netrw_up)
     " save netrw mapping
     let s:netrw_up = substitute(maparg('-', 'n'), '\c^:\%(<c-u>\)\=', '', '')