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

@@ -7,6 +7,8 @@ stages:
 
 check-commit-log:
   stage: review
+  variables:
+    GIT_DEPTH: "100"
   script:
     - ./.gitlab-ci/check-commit-log.sh
   only: