repair: Rename request_row_diff to get_row_diff
repair: Rename request_row_diff to get_row_diff

Make it consistent with the row level repair rpc verb.

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