[vim] Disable height calculation when 'preview' is found in the option string

Fix #2093

And we'll phase out height specification with `~` prefix.

README-VIM.md

@@ -128,7 +128,7 @@ let g:fzf_action = {
 
 " Default fzf layout
 " - down / up / left / right
-let g:fzf_layout = { 'down': '~40%' }
+let g:fzf_layout = { 'down': '40%' }
 
 " You can set up fzf window using a Vim command (Neovim or latest Vim 8 required)
 let g:fzf_layout = { 'window': 'enew' }
@@ -416,7 +416,7 @@ When fzf starts in a terminal buffer, the file type of the buffer is set to
 `fzf`. So you can set up `FileType fzf` autocmd to customize the settings of
 the window.
 
-For example, if you use the default layout (`{'down': '~40%'}`) on Neovim, you
+For example, if you use the default layout (`{'down': '40%'}`) on Neovim, you
 might want to temporarily disable the statusline for a cleaner look.
 
 ```vim

plugin/fzf.vim

@@ -629,6 +629,9 @@ function! s:calc_size(max, val, dict)
   endif
 
   let opts = $FZF_DEFAULT_OPTS.' '.s:evaluate_opts(get(a:dict, 'options', ''))
+  if opts =~ 'preview'
+    return size
+  endif
   let margin = match(opts, '--inline-info\|--info[^-]\{-}inline') > match(opts, '--no-inline-info\|--info[^-]\{-}\(default\|hidden\)') ? 1 : 2
   let margin += stridx(opts, '--border') > stridx(opts, '--no-border') ? 2 : 0
   if stridx(opts, '--header') > stridx(opts, '--no-header')