Fix not all .hs files are traversed (#821)

* Fix not all `.hs` files are traversed

* Check all tracked files only

.github/workflows/presubmit-cabal.yaml

@@ -73,4 +73,4 @@ jobs:
 
       # TODO(mihaimaruseac): Move to using a custom action that can be reused
       - name: Validate code formatting (self-validate)
-        run: cabal run hindent -- **/*.hs --validate
+        run: git ls-files -z '*.hs' | xargs -0 cabal run hindent -- --validate