ci: Ensure we clone a deep enough history for commit review

It seems gitlab changed something recently in the default clone depth
which made MRs with >10 commits to obscurely fail in the review stage.
As per https://docs.gitlab.com/ee/ci/yaml/#shallow-cloning, bump it
to 100 to allow bigger MRs.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/551

.gitlab-ci.yml

@@ -17,6 +17,8 @@ variables:
 check_commit_log:
     image: registry.gitlab.gnome.org/gnome/mutter/master:v1
     stage: review
+    variables:
+        GIT_DEPTH: "100"
     script:
         - ./.gitlab-ci/check-commit-log.sh
     only: