pkg-pull-updates: Fix tagging of imported commits after message trimming

Commit 4d11675a "pkg-pull-updates: Avoid oversized commit messages" resolved to amending the commit generated by gbp import-dsc to trim the overly long commit messages since they where causing issues with GitLab CI/CD.

However, gbp import-dsc also creates a git tag for the generated commit and after the amend the tag was left dangling, pointing to the original commit rather than the amended one.

Invoke gbp tag --retag to update the tag as well, so it get pushed again with the branch.