Fix jump to file for tree at root of commit

Closes https://github.com/tpope/vim-fugitive/issues/735

plugin/fugitive.vim

@@ -1408,7 +1408,9 @@ function! s:Edit(cmd,bang,...) abort
   catch /^fugitive:/
     return 'echoerr v:errmsg'
   endtry
+  if file !~# '^fugitive:'
     let file = s:sub(file, '/$', '')
+  endif
   if a:cmd ==# 'read'
     return 'silent %delete_|read '.s:fnameescape(file).'|silent 1delete_|diffupdate|'.line('.')
   else