update_changelog: if the new version does not exist yet, use HEAD

While there make sure that the result is consistent both when we
generate the changelog of a new version of Neubot and when we
generate the changelog of a previous, already-existing version.
scripts/release: automatically update the changelog

Historically I automatically updated the changelog, then I disabled
this feature, because I started making releases off master.

Now I stopped blessing releases off master, therefore it's handy
to restore the original behavior.
Neubot/0.4.16.7 released