mutation_fragment_merger: value-initialise iterator
mutation_fragment_merger: value-initialise iterator

ForwardIterators are default constructible, but they have to be
value-initialised to compare equal to other value-initialised instances
of that iterator.

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla