Inclusion loops rely only on href, not on xpointer

Just noticed that our loop detection can reject perfectly good
inclusions such as this one:

<xi:include xpointer="foo"/>
<x id="foo">
  <xi:include xpointer="bar"/>
</x>
<x id="bar"/>

The "bar" inclusion has the same include location (empty string) as the
foo inclusion.  We need to say loops are detected by comparing both the
include location and the xpointer attribute value.