Update PlugClean for Gist plugins

In order to install Gist as a plugin, you have to modify its `dir` to be
the `plugin` subdirectory of the normal path of the plugin if it were an
ordinary pathogen-compatible Vim plugin.

    Plug 'https://gist.github.com/952560a43601cd9898f1.git',
        \ { 'dir': g:plug_home.'/xxx/plugin', 'rtp': '..' }

PlugClean command was incompatible with the above trick, so it was
updated not to remove the parent directory of `dir` of each plugin.

plug.vim

@@ -1260,6 +1260,7 @@ function! s:clean(force)
 
   let allowed = {}
   for dir in dirs
+    let allowed[s:dirpath(fnamemodify(dir, ':h:h'))] = 1
     let allowed[dir] = 1
     for child in s:glob_dir(dir)
       let allowed[child] = 1