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

Author: simonjbeaumont

.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'