[vim] Use terminal buffer on 'down' layout on regular Vim on terminal

When 'down' layout was used on regular Vim on terminal, fzf would open
below the editor using `--height` option. This was the only case where
terminal buffer was not used (the code was written when Vim didn't have
builtin terminal) and this exception has been a constant source of
confusion.

This commit makes fzf open in a terminal buffer even in that case.

plugin/fzf.vim

@@ -487,17 +487,17 @@ try
   let has_vim8_term = has('terminal') && has('patch-8.0.995')
   let has_nvim_term = has('nvim-0.2.1') || has('nvim') && !s:is_win
   let use_term = has_nvim_term ||
-    \ has_vim8_term && !has('win32unix') && (has('gui_running') || s:is_win || !use_height && s:present(dict, 'down', 'up', 'left', 'right', 'window'))
+    \ has_vim8_term && !has('win32unix') && (has('gui_running') || s:is_win || s:present(dict, 'down', 'up', 'left', 'right', 'window'))
   let use_tmux = (has_key(dict, 'tmux') || (!use_height && !use_term || prefer_tmux) && !has('win32unix') && s:splittable(dict)) && s:tmux_enabled()
   if prefer_tmux && use_tmux
     let use_height = 0
     let use_term = 0
   endif
-  if use_height
+  if use_term
+    let optstr .= ' --no-height'
+  elseif use_height
     let height = s:calc_size(&lines, dict.down, dict)
     let optstr .= ' --height='.height
-  elseif use_term
-    let optstr .= ' --no-height'
   endif
   let optstr .= s:border_opt(get(dict, 'window', 0))
   let prev_default_command = $FZF_DEFAULT_COMMAND