DirCacheEditor: Replace file-with-tree and tree-with-file

DirCacheEditor: Replace file-with-tree and tree-with-file

If a PathEdit tries to store a file where a subtree was, or a subtree
where a file was, replace the entry in the DirCache with the new
name(s).  This supports switching between file and tree entry types
using a DirCacheEditor.

Add new unit tests to cover the conditions where these can happen.

Change-Id: Ie843d9388825f9e3d918a5666aa04e47cd6306e7