Patch to disable hidden cat during display of cat page
HIDDENCAT should not affect the display of Category namespace pages, otherwise you cannot navigate the category tree.
Note this affects 1.13-svn, but there is no option on bugzilla for that version.
I include a tested patch, which works on mediawiki svn r31239 (2008-02-24 18:09:10 -0500).
The patch uses the global variable wgTitle to get the current namespace. There may be a more localized way of doing this.