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

.github/workflows/pull_request_soundness.yml

@@ -126,9 +126,9 @@ jobs:
  - name: Mark the workspace as safe
  # 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 .
  - 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 '*.swift'
+ - name: Run format lint check
+ run: git ls-files -z '*.swift' | xargs -0 swift format lint --strict --parallel