d/gbp.conf: Set gbp-dch options matching existing changelog entries

When testing I try to build packages with a custom local pre-release version so it's obvious that it's not from/for the Debian archive, usually with gbp dch --commit --snapshot or ... --local="~local.". That led me to experiment a bit on matching the upstream changelog format, and here's that attempt.

There are two main differences from how it's currently done: this wraps lines to 70 characters instead of something closer to 80 (due to a python default), and it's not possible to directly sign off the git commit via gbp dch. To get a better sense of how the changelog differs, you can try:

• git checkout v0.143^
• git cherry-pick <this-commit>
• gbp dch --commit --release --distribution=experimental
• git diff HEAD v0.143