Consider this:

    $ wget http://www.thomas.schwinge.homeip.net/tmp/cutpaste_filter.tar.bz2
    $ wget http://www.thomas.schwinge.homeip.net/tmp/cutpaste_filter.patch
    
    $ tar -xj < cutpaste_filter.tar.bz2
    $ cd cutpaste_filter/
    $ ./render_locally
    $ find "$PWD".rendered/ -type f -print0 | xargs -0 grep -H -E 'FOO|BAR'
    [notice one FOO in there]
    $ rm -rf .ikiwiki "$PWD".rendered
    
    $ cp /usr/share/perl5/IkiWiki/Plugin/cutpaste.pm .library/IkiWiki/Plugin/
    $ patch -p0 < ../cutpaste_filter.patch
    $ ./render_locally
    $ find "$PWD".rendered/ -type f -print0 | xargs -0 grep -H -E 'FOO|BAR'
    [correct; notice no more FOO]

I guess this needs a general audit -- there are other places where `preprocess`
is being doing without `filter`ing first, for example in the same file, `copy`
function.

--[[tschwinge]]