pkg-pull-updates: 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 the
version from Bullseye on top of the Buster one.

GitLab then puts the whole log message in the `CI_COMMIT_MESSAGE` env
var, which is passed on the docker command line, resulting in a error:

    standard_init_linux.go:219: exec user process caused: argument list too long

See https://gitlab.com/gitlab-org/gitlab-runner/-/issues/26624#note_529234097



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

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