Here are a few suggestions for improving dpkg's handling of
configuration files which have been locally modified and also modified
by the package maintainer, during an upgrade.
Presently, the user is offered the choice between installing the
maintainer's (new) version, keeping the locally modified version,
viewing a diff between the two, and so on. It would be nice if it
were possible to also view a diff between the *former* maintainer's
version and the *new* maintainer's version, so the user could know
exactly which changes have been made by the maintainer and possibly
apply them locally. Furthermore, it would be useful to make a limited
automation available: the "diff3" utility exists exactly for this
reason, and it would be nice to have the option of running diff3 to
merge the differences between the old and new maintainer versions into
the locally modified file - and invoke an editor if there are
conflicts during the merge.
Lastly, it would be nice if it were possible to somehow flag a
configuration file as "locally rewritten from scratch", in which case
dpkg would automatically ignore it during upgrades, understanding that
it is entirely different from the maintainer's version and it would be
pointless to note changes in the latter (or perhaps just signal the
changes, but do not create a .dpkg-whatever file).