This is how I found the accidental bug fix for #3662:

    git show varnish-6.0.8:bin/varnishtest/tests/c00053.vtc >bisect.vtc
    tools/vtc-bisect.sh -g varnish-6.0.0 -j32 -i

I hope this will help others, I use this script whenever the cause of a
failure is not obvious.