[vim] Use 24-bit colors if termguicolors is set

plugin/fzf.vim

@@ -167,9 +167,12 @@ function! s:common_sink(action, lines) abort
 endfunction
 
 function! s:get_color(attr, ...)
+  let gui = has('termguicolors') && &termguicolors
+  let fam = gui ? 'gui' : 'cterm'
+  let pat = gui ? '^#[a-f0-9]\+' : '^[0-9]\+$'
   for group in a:000
-    let code = synIDattr(synIDtrans(hlID(group)), a:attr, 'cterm')
-    if code =~ '^[0-9]\+$'
+    let code = synIDattr(synIDtrans(hlID(group)), a:attr, fam)
+    if code =~? pat
       return code
     endif
   endfor