reinstalling local deb file is no downgrade

reinstalling local deb file is no downgrade

If we have a (e.g. locally built) deb file installed and do try to
install it again apt complained about this being a downgrade, but it
wasn't as it is the very same version… it was just confused into not
merging the versions together which looks like a downgrade then.

The same size assumption is usually good, but given that volatile files
are parsed last (even after the status file) the base assumption no
longer holds, but is easy to adept without actually changing anything in
practice.

(cherry picked from commit e7edb2fef8370d54a4b8e5a01266e6eda81ef84e)