Fix parallel sort, broken by the balanced merge patch.

The code for initializing the tapes on each merge iteration was skipped in a parallel worker. I put the !WORKER(state) check in wrong place while rebasing the patch. That caused failures in the index build in 'multiple-row-versions' isolation test, in multiple buildfarm members. On my laptop it was easier to reproduce by building an index on a larger table, so that you got a parallel sort more reliably.

Author: hlinnaka

src/backend/utils/sort/tuplesort.c

@@ -2923,7 +2923,7 @@ mergeruns(Tuplesortstate *state)
  * Rewind all the output tapes, and make them inputs for the next
  * pass.
  */
-if (state->nInputRuns == 0 && !WORKER(state))
+if (state->nInputRuns == 0)
 {
 int64input_buffer_size;
 
@@ -2975,7 +2975,8 @@ mergeruns(Tuplesortstate *state)
  * sorted tape, we can stop at this point and do the final merge
  * on-the-fly.
  */
-if (!state->randomAccess && state->nInputRuns <= state->nInputTapes)
+if (!state->randomAccess && state->nInputRuns <= state->nInputTapes
+&& !WORKER(state))
 {
 /* Tell logtape.c we won't be writing anymore */
 LogicalTapeSetForgetFreeSpace(state->tapeset);