passed extend = true whenever we are reading a page we intend to reinitialize
completely, even if we think the page "should exist".  This is because it
might indeed not exist, if the relation got truncated sometime after the
current xlog record was made and before the crash we're trying to recover
from.  These two thinkos appear to explain both of the old bug reports
discussed here:
http://archives.postgresql.org/pgsql-hackers/2005-05/msg01369.php