shada: Refactor history merger to use statically sized 2linked list

Also fixes the error: when writing shada files existing history items may be
freed.

Warning: valgrind reports some memory leaks.

src/nvim/api/private/helpers.c

@@ -597,7 +597,7 @@ static void init_type_metadata(Dictionary *metadata)
 }
 
 /// Creates a deep clone of an object
-static Object copy_object(Object obj)
+Object copy_object(Object obj)
 {
   switch (obj.type) {
     case kObjectTypeNil: