Revert "gitlab-ci: remove git fetch before_script"

Decided it's better to do it in one place than repeating the logic on
multiple places.

This reverts commit fb5b97d0.

.gitlab-ci.yml

@@ -6,6 +6,13 @@ variables:
   GIT_STRATEGY: fetch
   GIT_DEPTH: "0"
 
+default:
+    # Make sure master points to the correct upstream commit
+    before_script:
+      - >
+        git fetch $CI_MERGE_REQUEST_PROJECT_URL
+        +refs/heads/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME:refs/heads/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME
+
 lint:
   stage: lint
   image: alpinelinux/apkbuild-lint-tools:latest