Adds heuristic CI test to prevent merges that remove lines

.gitlab-ci.yml

@@ -6,6 +6,9 @@ test:
   script:
     - javac *.java
     - java GenerateSite
+    # heuristic to see if any lines were removed!
+    - git fetch origin main
+    - '(exit $(git diff origin/main HEAD | grep "^-[^-]" | wc -l)) || { echo "Malformed merge request: at least one line is removed in diff." && (exit 1) }'
 
 pages:
   tags: