feat(commitlint): users can configure the base branch

Users can override the variable `BASE_BRANCH` in their
`.gitlab-ci.yml` to select the starting point of the logs to
validate.

templates/Lint/Commitlint.yaml

@@ -7,10 +7,11 @@ commitlint:
   image: "$COMMITLINT_IMAGE"
   variables:
     COMMITLINT_IMAGE: 'hub.eole.education/eole/commitlint:latest'
+    BASE_BRANCH: "${CI_DEFAULT_BRANCH}"
   script:
     - 'git fetch --all'
     # Set default commit hashes for `--from` and `--to`
-    - 'export COMMITLINT_FROM="$(git merge-base origin/${CI_DEFAULT_BRANCH} HEAD)"'
+    - 'export COMMITLINT_FROM="$(git merge-base origin/${BASE_BRANCH} HEAD)"'
     - 'export COMMITLINT_TO="${CI_COMMIT_SHA}"'
     # Run `commitlint`
     - 'commitlint --from "${COMMITLINT_FROM}"