/etc/apt/apt.conf.d/20listdifferences is in apt-listdifferences 1.20170813.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | DPkg::Pre-Install-Pkgs { "test -x /usr/sbin/apt-listdifferences || exit 0; /usr/sbin/apt-listdifferences --apt || true"; };
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