pgsql: logtape.c: do not preallocate for tapes when sorting

logtape.c: do not preallocate for tapes when sorting

The preallocation logic is only useful for HashAgg, so disable it when
sorting.

Also, adjust an out-of-date comment.

Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzn_o7tE2+hRVvwSFghRb75AJ5g-nqGzDUqLYMexjOAe=g@mail.gmail.com
Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/93106d71a18afdda2b9bf6e6b8e6c7f9cea2d0ef

Modified Files
--------------
src/backend/executor/nodeAgg.c     |  2 +-
src/backend/utils/sort/logtape.c   | 38 +++++++++++++++++++++++++-------------
src/backend/utils/sort/tuplesort.c |  7 ++++---
src/include/utils/logtape.h        |  3 ++-
4 files changed, 32 insertions(+), 18 deletions(-)