repair: Avoid copying position in to_repair_rows_list

No need to make a copy because it is not used to construct repair_row
any more since commit 9079790 (repair:
Avoid writing row with same partition key and clustering key more than
once). Use mf->position() instead.

Refs: scylladb#4510
Backports: 3.1
Message-Id: <7b21edcc3368036b6357b5136314c0edc22ad4d2.1560753672.git.asias@scylladb.com>

repair/row_level.cc

@@ -1055,13 +1055,12 @@ class repair_meta {
                         _metrics.rx_row_nr += 1;
                         _metrics.rx_row_bytes += fmf.representation().size();
                         auto mf = make_lw_shared<mutation_fragment>(fmf.unfreeze(*_schema));
-                        position_in_partition pos(mf->position());
                         // If the mutation_fragment has the same position as
                         // the last mutation_fragment, it means they are the
                         // same row with different contents. We can not feed
                         // such rows into the sstable writer. Instead we apply
                         // the mutation_fragment into the previous one.
-                        if (last_mf && cmp(last_mf->position(), pos) == 0 && last_mf->mergeable_with(*mf)) {
+                        if (last_mf && cmp(last_mf->position(), mf->position()) == 0 && last_mf->mergeable_with(*mf)) {
                             last_mf->apply(*_schema, std::move(*mf));
                         } else {
                             last_mf = mf;