Convert checkout to a regular action (#70)

__test__/verify-clean.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+if [[ "$(git -C ./basic status --porcelain)" != "" ]]; then
+    echo ----------------------------------------
+    echo git status
+    echo ----------------------------------------
+    git status
+    echo ----------------------------------------
+    echo git diff
+    echo ----------------------------------------
+    git diff
+    exit 1
+fi