Fix tree breaking on directory listings

[bprb]

This fixes an error when a buffer contains a folder and another buffer contains a file in that folder.

Example repro steps:

1. open c:\myfolder in a buffer (a directory listing)
2. open c:\myfolder\foo.txt in a buffer
3. run BufferTree

In the first iteration of GetTree, the folder will be added as a buffer_number, not as a dictionary. The second iteration then throws an error when trying to use it as a dictionary in let dir = dir[step].

The fix is to add directories as dictionaries, always.

[bprb]

Example of the issue:

:ls
  3  h=  "."                            line 1
  4 %a   "test.vim"                     line 1

:BufferTree
Error detected while processing function tree#BufferTree[3]..GetTree:
line   12:
E689: Can only index a List, Dictionary or Blob
└─ C:/Users/username ⇒ 3