Record References: fix copy and assignment handling

not entirely sure about the design, but lets try this approach:
they can be "cloned" and likewise move-assigned, but we do not
allow the regular assignment, because this would enable to use
references like pointers (what we deliberately do not want)

src/lib/diff/record.hpp

@@ -514,7 +514,20 @@ namespace diff{
       /** prevent moving into black hole */
       RecordRef(Target&&) = delete;
       
-      // standard copy operations acceptable
+      RecordRef(RecordRef const&)  = default;
+      RecordRef(RecordRef     &&)  = default;
+      
+      /** references can not be reassigned */
+      RecordRef& operator= (RecordRef const&)  = delete;
+      RecordRef& operator= (RecordRef      &)  = delete;
+      
+      /** assignment is not allowed, but moving is */
+      RecordRef&
+      operator= (RecordRef &&o)
+        {
+          std::swap(record_, o.record_);
+          return *this;
+        }
       
       
       explicit

tests/library/diff/generic-record-representation-test.cpp

@@ -326,12 +326,13 @@ namespace test{
           CHECK ("🌰" == oor.getType());
           CHECK (oor.get("♄") == "saturn");
           
-          // are copyable and assignable
+          // are copyable but not reassignable
           RecordRef<string> r2 = ref;
           CHECK (r2);
           CHECK (r2.get() == ref.get());
           CHECK (!isSameObject (r2, ref));
           
+          // but references are move-assignable
           empty = std::move(r2);
           CHECK (empty);
           CHECK (!r2);