CI: Reduce cpplint output to only changed lines

Author: chennes

.github/workflows/sub_lint.yml

@@ -586,7 +586,11 @@ jobs:
           if [ ${{ env.logdir }}cpplint.log ]
           then
             echo "::add-matcher::${{ runner.workspace }}/FreeCAD/.github/problemMatcher/cpplint.json"
-            cat ${{ env.logdir }}cpplint.log
+          
+            # Only show lint on changed lines:
+            git diff -U0 origin/${{ github.event.pull_request.base.ref }} | grep -E '^\+\s' | sed 's/^+//' > changed_lines.txt
+            grep -Ff changed_lines.txt ${{ env.logdir }}cpplint.log || true
+          
             echo "::remove-matcher owner=cpplint::"
             cpplintErrors=$(grep -nIHE "\[[0-9]\]$" ${{ env.logdir }}cpplint.log | wc -l ) || true
           fi