pgsql: Adjust tuplesort.c based on the fact that we never use the OS's

Adjust tuplesort.c based on the fact that we never use the OS's qsort().

Our own qsort_arg() implementation doesn't have the defect previously
observed to affect only QNX 4, so it seems sufficiently to assert that
it isn't broken rather than retesting.  Also, update a few comments to
clarify why it's valuable to retain a tie-break rule based on CTID
during index builds.

Peter Geoghegan, with slight tweaks by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c5a03256c725c09c32a5c498bd7c8799ed3ec2a0

Modified Files
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src/backend/utils/sort/tuplesort.c |   24 ++++++++++++------------
1 files changed, 12 insertions(+), 12 deletions(-)