fix logic bug in existing tree diff applicator

this one went through unnoticed, because the situation
is not covered in unit-test. The tests written thus fare
are more like a proof-of-concept. I didn't want to spend
weeks on writing extensive coverage of all corner cases,
at least not before all aspects of the tree diff protocol
are settled. Seemingly this backfires already

src/lib/diff/tree-diff-application.hpp

@@ -371,7 +371,10 @@ namespace diff{
           __expect_successful_location(n);
           
           if (not endOfData() and srcPos()->matches(n))
-            ++src(); // get /after/ an explicitly given position
+            {
+              move_into_new_sequence (srcPos());
+              ++src(); // get /after/ an explicitly given position
+            }
         }
       
       /** assignement of changed value in one step */