If Diff cannot find a revision the NoSuchRevision()
function in stdlib.php is called, which in turn calls
GeneratePage(). However GeneratePage() is not defined
and a error message is displayed. This occurs due to
the problem with pageinfo mentioned in another bug report.
The GeneratePage() function of relevence seems to be
defined in Template.php so the attached patch includes
this file. This may not be the best way to do it, but
it works for me.