private: Add utility to generate the tag summary

Signed-off-by: Niels Thykier <niels@thykier.net>
d/changelog: Add reminder to add tag summary

Now that we have a tool to generate the tag summary, leave a marker to
do it before release.  Furthermore, have a test remind us if we are
about to release without removing the TODO marker.

Signed-off-by: Niels Thykier <niels@thykier.net>