Switch to script-based git status modified check

.github/workflows/check_formatting.yml

@@ -24,4 +24,4 @@ jobs:
         run: npm run format
 
       - name: Check for dirty working tree
-        run: git diff | grep -q 'modified' && exit 1 || exit 0
+        run: scripts/check-dirty-tree.sh

scripts/check-dirty-tree.sh

@@ -0,0 +1,19 @@
+#! /bin/bash
+
+#####################################################################
+# Per https://sharats.me/posts/shell-script-best-practices/ (partial)
+set -o errexit
+set -o nounset
+set -o pipefail
+
+if [[ "${TRACE-0}" == "1" ]]; then
+    set -o xtrace
+fi
+#####################################################################
+
+git status | grep modified:
+
+if [ $? = 0 ]
+then
+  exit 1
+fi