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.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>