Set 'exit_code' to 'error_on_format' and batch formatting

Author: wenkokke

.stylish-haskell.yaml

@@ -511,3 +511,6 @@ language_extensions:
 # We run stylish from the root of the repo which is a project so this flag
 # doesn't apply
 cabal: false
+
+# Return an exit failure on format.
+exit_code: error_on_format

scripts/format-stylish.sh

@@ -18,6 +18,6 @@ fi
 # Check Haskell files with stylish-haskell
 echo "Formatting Haskell source files with stylish-haskell version ${stylish_haskell_required_version}"
 # shellcheck disable=SC2016
-if ! git ls-files --exclude-standard --no-deleted --deduplicate '*.hs' | xargs -L 1 sh -c 'echo "$0" && stylish-haskell -i -c .stylish-haskell.yaml "$0"'; then
+if ! git ls-files --exclude-standard --no-deleted --deduplicate '*.hs' | xargs -L 50 stylish-haskell -i -c .stylish-haskell.yaml; then
     exit 1
 fi