make checkout version and revert version fit pattern (#118)

src/lib/assert.ts

@@ -0,0 +1,8 @@
+export function assertExists<T>(
+  value: T,
+  message: string,
+): asserts value is NonNullable<T> {
+  if (value === undefined || value === null) {
+    throw new Error(message);
+  }
+}