RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^wiki/(.*)$ mediawiki/index.php?title=$1 [PT,L,QSA]
RewriteRule ^wiki/*$ wiki/ [L,QSA]
RewriteRule ^/*$ wiki/ [L,QSA]