fix base branch detection in fuzzer jobs

Author: strtgbb

docker/test/fuzzer/run-fuzzer.sh

@@ -50,7 +50,28 @@ function clone
                 git checkout "$SHA_TO_TEST"
                 echo "Checked out nominal SHA $SHA_TO_TEST for PR $PR_TO_TEST"
             fi
-            git diff --name-only master HEAD | tee ci-changed-files.txt
+            # Get the base branch from PR metadata using GitHub CLI
+            git fetch --depth 1 origin
+
+            BASE_BRANCH=$(gh pr view "$PR_TO_TEST" --json baseRefName --jq '.baseRefName' 2>/dev/null || echo "")
+            if [ -z "$BASE_BRANCH" ]; then
+                echo "ERROR: Could not determine base branch for PR $PR_TO_TEST"
+                echo "Make sure GitHub CLI (gh) is available and you have access to the repository"
+                exit 1
+            fi
+
+            echo "Found base branch from PR metadata: $BASE_BRANCH"
+            MERGE_BASE=$(git merge-base HEAD "origin/$BASE_BRANCH" 2>/dev/null || echo "")
+
+            # Exit with error if no merge base found
+            if [ -z "$MERGE_BASE" ]; then
+                echo "ERROR: No merge base found with base branch: $BASE_BRANCH"
+                echo "Available remote branches:"
+                git branch -r
+                exit 1
+            else
+                git diff --name-only "$MERGE_BASE" HEAD | tee ci-changed-files.txt
+            fi
         else
             if [ -v SHA_TO_TEST ]; then
                 git fetch --depth 2 origin "$SHA_TO_TEST"