fix php warning in common.php during getRevisions

A php warning can occur when the page name used in a revision check clashes with
a namespace (sub-directory) in the same directory.  The strpos() call generates
the warning as the offset used is longer than the directory name.

darcs-hash:20060601212928-9b6ab-2d9399ba71fbfcae6e797977476e2f5daa50cf7f.gz