gitlab-ci: add whitespace error check

gitlab-ci: add whitespace error check

GitLab CI does not have a job to check for whitespace errors introduced
by a set of changes. Reuse the existing generic `whitespace-check.sh` to
create the job for GitLab pipelines.

Note that the `$CI_MERGE_REQUEST_TARGET_BRANCH_SHA` variable is only
available in GitLab merge request pipelines and therefore the CI job is
configured to only run as part of those pipelines.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>