[neovim] Do not use Pmenu group colors for floating window

In Neovim, the foreground and background colors of a floating window
defaults to those of Pmenu highlight group, which yields unexpected
results.

This commit makes the colors of fzf window defaults to those of 'Normal'
group (or 'NormalFloat' if defined), by ignoring Pmenu group.

Then the colors can be configured via --color option of fzf.

NOTE: An error from setwinvar call is ignored because the exact
behavior of &winhighlight with an empty target group is not clearly
documented.

Close #3035
Close https://github.com/junegunn/fzf.vim/issues/1431
See https://github.com/neovim/neovim/pull/9722#discussion_r264777602

plugin/fzf.vim

@@ -973,16 +973,16 @@ function! s:callback(dict, lines) abort
 endfunction
 
 if has('nvim')
-  function s:create_popup(hl, opts) abort
+  function s:create_popup(opts) abort
     let buf = nvim_create_buf(v:false, v:true)
     let opts = extend({'relative': 'editor', 'style': 'minimal'}, a:opts)
     let win = nvim_open_win(buf, v:true, opts)
-    call setwinvar(win, '&winhighlight', 'NormalFloat:'..a:hl)
+    silent! call setwinvar(win, '&winhighlight', 'Pmenu:')
     call setwinvar(win, '&colorcolumn', '')
     return buf
   endfunction
 else
-  function! s:create_popup(hl, opts) abort
+  function! s:create_popup(opts) abort
     let s:popup_create = {buf -> popup_create(buf, #{
       \ line: a:opts.row,
       \ col: a:opts.col,
@@ -1017,7 +1017,7 @@ function! s:popup(opts) abort
   let row += !has('nvim')
   let col += !has('nvim')
 
-  call s:create_popup('Normal', {
+  call s:create_popup({
     \ 'row': row, 'col': col, 'width': width, 'height': height
   \ })
 endfunction