[GHA] Only format Swift files that are in Git index (#2797)

.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
+ - name: Format Swift files
+ run: git ls-files -z '*.swift' | xargs -0 swift format format --parallel --in-place
+ - name: Lint Swift files
+ run:  git ls-files -z '*.swift' | xargs -0 swift format lint --strict --parallel
+ - name: Check format produced no diff
+ run: GIT_PAGER= git diff --exit-code '*.swift'