[vim] Use 24-bit colors on gvim even when &termguicolors is off

Close #2563

Author: junegunn

plugin/fzf.vim

@@ -358,7 +358,7 @@ endfunction
 
 function! s:get_color(attr, ...)
   " Force 24 bit colors: g:fzf_force_termguicolors (temporary workaround for https://github.com/junegunn/fzf.vim/issues/1152)
-  let gui = get(g:, 'fzf_force_termguicolors', 0) || (!s:is_win && !has('win32unix') && has('termguicolors') && &termguicolors)
+  let gui = get(g:, 'fzf_force_termguicolors', 0) || (!s:is_win && !has('win32unix') && (has('gui_running') || has('termguicolors') && &termguicolors))
   let fam = gui ? 'gui' : 'cterm'
   let pat = gui ? '^#[a-f0-9]\+' : '^[0-9]\+$'
   for group in a:000