I think that
apt isn't able to produce such a list, per se. The question
apt seems to prompt for is based on the results of
dpkg that knows what files are going to be updated and checking those files for local changes (how, I don't know).
The downside of using only those changes in e.g. documenting a server is, that it only reports changes made to actual and original config files from packages (usually in
/etc/.../<prog>.conf), not users' personal configs (
~/.<prog>.conf) or such a packages that use overridable config files (
Disclaimer: I haven't dug into the deep internals of
dpkg so I might very well have gotten some detail wrong. Corrections are welcome.
A few tips still to ease the preparations for similar situations in the future:
There is a program called etckeeper that is specifically designed to track changes to configuration files in
/etc/. It basically keep all files in VCS, enhanced with permission and metadata tracking. Tracking is hooked into various packet management systems so that any new or removed packages trigger the configuration to be saved.
Naturally, this won't help in your situation, where and if
etckeeper isn't already installed and tracking changes to the environment right from the start.
dpkg-changes is a perl script to track changes in installed/removed packages, but it doesn't keep a log of individual files introduced by those packages.