gitlab: skip checkpatch.pl checks if no commit delta on branch

If the current branch is synced to the current upstream git master,
there are no commits that need checking. This causes checkpatch.pl
to print an error that it found no commits. We need to avoid calling
checkpatch.pl in this case.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201019143537.283094-2-berrange@redhat.com>
Message-Id: <20201021163136.27324-5-alex.bennee@linaro.org>