bugfix for the e mapping

if we were using e on the current dir (or a file in that dir) then we
would end up doing an ":edit" with no target. Now we use "." in
Path#strForEditCmd if it would resolve to an empty string

plugin/NERD_tree.vim

@@ -1567,6 +1567,10 @@ function! s:Path.strForEditCmd()
         let p = strpart(p, strlen(cwd)+1)
     endif
     
+    if p == ''
+        let p = '.'
+    endif
+
     return p
 
 endfunction