pgsql: Improve performance of our private version of qsort.

Log Message:
-----------
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.

Tags:
----
REL8_1_STABLE

Modified Files:
--------------
    pgsql/src/port:
        qsort.c (r1.8 -> r1.8.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/qsort.c.diff?r1=1.8&r2=1.8.2.1)