import-debian-package: avoid oversized commit messages

gbp import-dsc puts all the new changelog entries in the commit message, generating big walls of text when, for instance, importing a new package for the first time. GitLab then puts the whole log message in the CI_COMMIT_MESSAGE env var, which makes the runner to fail during the executor preparation section with the error:

exec /usr/bin/dumb-init: argument list too long

See: https://gitlab.com/gitlab-org/gitlab/-/issues/392406

To avoid that, trim the message to only keep the first line.

Fix: infrastructure/apertis-issues#395 (closed)