Hi,

we're using mini-dinstall to vendor some packages from upstream releases
that aren't distributed as part of Debian. We recently ran into an issue
with an upstream (vector.dev) that publishes debian packages under the
naming scheme `<package>-<version>-<arch>.deb`. When processing a
changes file that references these files, the mini-dintall logs look
like the installation succeeded:

INFO: Examining "incoming/vector_0.20.0_amd64_buster.changes"
INFO: Preparing to install "incoming/vector_0.20.0_amd64_buster.changes" in archive buster
INFO: Successfully installed vector 0.20.0 to buster

However, only the change file is copied over; the debian package remains
in the incoming directory.

In #590305, Joey Hess suggested that mini-dinstall could install such
"unknown" files instead of skippint them.  Regardless, it would be
really useful if mini-dinstall at least printed a warning when it
ignores files in the changes directory.

If I manage to sign up for an account on salsa.debian.org, I'd be happy
to submit a merge request to print a warning. Let me know if this is
acceptable, this is the first time I'm contributing to Debian.