import-dsc; Merge upstream version by tag

instead of simply using the upstream branch name. THis makes sure
we also merge upstream versions that were imported previously with
e.g. "gbp import-orig".

Closes: #698222
log: add error and warning aliases

since I tend to use them instead of err and warn.

Closes: #728896
Fiz description of backports branches
Use --distribution instead of --dist

Closes: #730788
Document changes and release 0.6.7