Fix links generated by preprocessor directives when previewing.

As was already done for linkfication, links generated in a prevew page
are relative to the top of the wiki, so it has to be told that the destpage
is there.

I was using "" to indicate this, but that may confuse some preprocessor
plugins, which treat parameters with an empry value specially (sparkline is one
such). Instead, use "/", which is more accurate anyway and works just as well.