[GHA] Only format Swift files that are in Git index

Author: simonjbeaumont

.github/workflows/pull_request_soundness.yml

@@ -127,8 +127,8 @@ jobs:
     # https://github.com/actions/checkout/issues/766
       run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
     - name: Run format lint check
-      run:  swift format lint --strict --recursive --parallel .
+      run:  git ls-files -z '*.swift' | xargs -0 swift format lint --strict --parallel
     - name: Run format and check for modified files
       run: |
-        swift format format --parallel --recursive --in-place .
-        git diff-index --quiet HEAD
+        git ls-files -z '*.swift' | xargs -0 swift format format --parallel --in-place
+        GIT_PAGER= git diff --exit-code