Tests whether the DifferenceEngine is currently processing
comparisons of "things" between the document node and the
document's root element (comments or PIs, mostly) since these
must be ignored for backwards compatibility reasons.
Relies on the following assumptions:
- the last comparison DOMDifferenceEngine performs on the
document node is an XML_ENCODING comparison.
- the first comparison DOMDifferenceEngine performs on matching
root elements is a NODE_TYPE comparison. The control Node
is an Element Node.
- the first comparison DOMDifferenceEngine performs if the
root elements don't match is a CHILD_LOOKUP comparison.
The control Node is an Element Node.