(bug 4220) dump schema get unique contraints on id

(bug 4220) dump schema get unique contraints on id

This patch was made in December 2005 by Elvis Stansvik on bug 4220, you
will find below the rewritten introduction for his patch contribution.

"""
The XML Schema for the XML dump format used by MediaWiki has no
constraints for the page and revision identifiers. This patch solve that
issue by enforcing the constraint in the XSD.

It most probably makes sense, since I think that:

 - some parsers capable of Schema validation can work more efficiently
   if constraints are in the XSD.
 - Another reason is that (however unlikely) some other software might
   output files in this format are not obliged to keep the IDs unique,
   according the the XSD in its current form.
"""

Updating the http://www.mediawiki.org/xml/export-0.7/ public URL is
tracked by bug 37111.

Change-Id: Idb82fe620c3395acaa8f051b4d4d0d770cd33d44