fix diff round-trip comparison

tests/postgres/issue21/spec.dbml

@@ -0,0 +1,28 @@
+Table users {
+    id integer [pk, not null]
+    username varchar(255) [not null]
+    email varchar(255) [not null]
+    created_at timestamptz [not null]
+    profile_id integer
+}
+
+Table profiles {
+    id integer [pk, not null]
+    bio text
+    created_at timestamptz [not null]
+}
+
+Table posts {
+    id integer [pk, not null]
+    user_id integer [not null]
+    title varchar(255) [not null]
+    body text
+    published_at timestamptz
+    view_count integer default 0
+
+    Indexes {
+        (user_id, title) [unique]
+    }
+}
+
+Ref: users.profile_id > profiles.id