feat: migrate out of the flat store layout

tests/link_store.rs

@@ -306,3 +306,44 @@ fn a_link_into_an_older_store_layout_still_checks_clean_and_restores() {
         .failed()
         .says("not in the store");
 }
+
+#[test]
+fn migrate_moves_only_this_checkouts_paths_out_of_the_old_layout() {
+    let case = Case::new("migrate");
+    let old = case.store.parent().unwrap().to_path_buf();
+    fs::create_dir_all(old.join("secrets")).unwrap();
+    fs::write(old.join(".env"), b"SECRET=1\n").unwrap();
+    fs::write(old.join("secrets/token"), b"tok\n").unwrap();
+    // another checkout's file in the same flat layout, linked from nowhere here
+    fs::write(old.join("other.env"), b"OTHER\n").unwrap();
+    case.link(&case.path(".env"), &old.join(".env"));
+    case.link(&case.path("secrets"), &old.join("secrets"));
+    case.gitignore(".env\nsecrets/\n");
+
+    case.ahab(&["link", "list"])
+        .ok()
+        .says("legacy:    .env")
+        .says("legacy:    secrets")
+        .says("ahab link migrate");
+    case.ahab(&["status"]).says("legacy:    2");
+
+    case.ahab(&["link", "migrate"]).ok().says("migrated:  .env");
+
+    assert_eq!(
+        fs::read_link(case.path(".env")).unwrap(),
+        case.store.join(".env")
+    );
+    assert_eq!(fs::read(case.path(".env")).unwrap(), b"SECRET=1\n");
+    assert_eq!(fs::read(case.path("secrets/token")).unwrap(), b"tok\n");
+    assert!(!old.join(".env").exists());
+    assert!(!old.join("secrets").exists());
+    assert_eq!(fs::read(old.join("other.env")).unwrap(), b"OTHER\n");
+
+    case.ahab(&["link", "list"])
+        .ok()
+        .says("linked:    .env")
+        .silent_about("legacy");
+    case.ahab(&["link", "migrate"])
+        .ok()
+        .says("nothing in the old layout");
+}