Fix broken comparetup_datum code.

Commit 337b6f5ecf05b21b5e997986884d097d60e4e3d0 contained the entirely
fanciful assumption that it had made comparetup_datum unreachable.
Reported and patched by Takashi Yamamoto.

Fix up some not terribly accurate/useful comments from that commit, too.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/17b985b1a068e120feb05bb61be6328f4fef8090

Modified Files
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src/backend/utils/sort/tuplesort.c |   20 ++++++++++++--------
1 files changed, 12 insertions(+), 8 deletions(-)

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