repair: Add get_row_diff_source_op
repair: Add get_row_diff_source_op

It is a helper that works on the source() of the
get_row_diff rpc stream verb.

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