pgsql: Improve performance of our private version of qsort.

Log Message:
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Improve performance of our private version of qsort. Per recent testing,
the logic it contained to switch to insertion sort for near-sorted input was
in fact a big loss, because it could fairly easily be fooled into applying
insertion sort to large subfiles that weren't all that well ordered. Remove
that, and instead add a simple check for already-perfectly-sorted input, as
per suggestion from Dann Corbit. This adds at worst O(N*lgN) overhead, and
usually far less, while sometimes allowing a subfile sort to finish in O(N)
time. Preliminary testing says this is an improvement over the basic
Bentley & McIlroy code for many nonrandom inputs, and it costs almost
nothing when the input is random.

Modified Files:
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pgsql/src/port:
qsort.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/qsort.c.diff?r1=1.8&r2=1.9)