This makes 'redo -j1000' now run successfully in t/curse, except that we
foolishly generate the same files more than once. But at least not more
than once *in parallel*.
The problem is that redo-ifchange has a different $PWD than its
sub-dependencies, so as it's chasing them down, fixing up the relative paths
totally doesn't work at all.
There's probably a much smarter fix than this, but it's too late at night to
think of it right now.
