Fixed some trailing slash bugs (#957)

* Fixed issue where user would have to trigger dir-up action multiple times to get it to work

* Fix to path_join to ensure the result does not have a double slash

lua/nvim-tree/actions/dir-up.lua

@@ -1,10 +1,12 @@
+local utils = require'nvim-tree.utils'
+
 local M = {}
 
 function M.fn(node)
   if not node or node.name == ".." then
     return require'nvim-tree.actions.change-dir'.fn('..')
   else
-    local newdir = vim.fn.fnamemodify(TreeExplorer.cwd, ':h')
+    local newdir = vim.fn.fnamemodify(utils.path_remove_trailing(TreeExplorer.cwd), ':h')
     require'nvim-tree.actions.change-dir'.fn(newdir)
     return require"nvim-tree.actions.find-file".fn(node.absolute_path)
   end