Have a defined result for Tree.path_content_summary() for unversioned files.
WT.path_content_summary() was failing because it was trying to look up the object in the working inventory, and that has no record for unversioned content. PreviewTree seems to be a bit indeterminant. Aaron claims it should also return a 'file' and the info it knows about, but it currently returns 'missing'. That is now marked as a 'knownFailure'.