Support browsing with new netrw.vim

The previous fix for #594 didn't handle the case in which the net netrw
was installed on older versions of Vim.

plugin/fugitive.vim

@@ -2225,7 +2225,10 @@ function! s:Browse(bang,line1,count,...) abort
     elseif exists(':Browse') == 2
       return 'echomsg '.string(url).'|Browse '.url
     else
-      if has("patch-7.4.567")
+      if !exists('g:loaded_netrw')
+        runtime! autoload/netrw.vim
+      endif
+      if exists('*netrw#BrowseX')
         return 'echomsg '.string(url).'|call netrw#BrowseX('.string(url).', 0)'
       else
         return 'echomsg '.string(url).'|call netrw#NetrwBrowseX('.string(url).', 0)'