dh_installchangelogs: Trim changelogs by default

Changelogs of older and heavily developed packages can reach sizes of hundreds of kilobytes. Even after compression, such big changelogs are a waste of space and bandwidth worldwide.

These commits make dh_installchangelog trim changelogs to include only entries more recent than the release date of oldstable (currently Debian 10 Buster), unless the --no-trim option is passed.

(This MR is a reduced version of MR !79 (closed) where the trimming happens by default and the cutoff date is set manually.)