vim: update termguicolor restrictions

Require Neovim v0.3.2+.
neovim/neovim@98eaf60
Require Vim v8.1.0839+ with +vcon feature on Windows.
vim/vim@f58d81a

Author: janlazo

shared.vim

@@ -371,10 +371,17 @@ if has('syntax')
   endif
 
   function! s:set_color()
-    if has('termguicolors') &&
-    \  (has('nvim') || !has('win32') || !has('patch-8.0.1531') || has('vcon'))
-      let &termguicolors = &t_Co == 256 && empty($TMUX) && !has('osx') &&
-      \ (has('nvim') || has('patch-8.0.0146'))
+    if has('termguicolors')
+    \ && (has('nvim')
+          \ || !has('win32')
+          \ || !has('patch-8.0.1531')
+          \ || has('vcon'))
+      let &termguicolors = &t_Co == 256 && empty($TMUX) && !has('osx')
+      \ && (has('nvim')
+            \ ? has('nvim-0.3.2')
+            \ : (has('win32')
+                \ ? (has('vcon') && has('patch-8.1.0839'))
+                \ : has('patch-8.0.0146')))
     endif
 
     if s:is_gui