Fix restoring alternate terminal with completeopt redirection

Fixes https://github.com/davidhalter/jedi-vim/issues/721.

plugin/jedi.vim

@@ -16,11 +16,28 @@ if get(g:, 'jedi#auto_vim_configuration', 1)
     filetype plugin on
 
     " Change completeopt, but only if it was not set already.
-    redir => completeopt
-        silent verb set completeopt?
-    redir END
-    if len(split(completeopt, '\n')) == 1
-        set completeopt=menuone,longest,preview
+    " This gets done on VimEnter, since otherwise Vim fails to restore the
+    " screen.  Neovim is not affected, this is likely caused by using
+    " :redir/execute() before the (alternate) terminal is configured.
+    function! s:setup_completeopt()
+        if exists('*execute')
+            let completeopt = execute('silent verb set completeopt?')
+        else
+            redir => completeopt
+                silent verb set completeopt?
+            redir END
+        endif
+        if len(split(completeopt, '\n')) == 1
+            set completeopt=menuone,longest,preview
+        endif
+    endfunction
+    if has('nvim')
+        call s:setup_completeopt()
+    else
+        augroup jedi_startup
+            au!
+            autocmd VimEnter * call s:setup_completeopt()
+        augroup END
     endif
 
     if len(mapcheck('<C-c>', 'i')) == 0