pgsql: Since sort_bounded_heap makes state changes that should be made

Log Message:
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Since sort_bounded_heap makes state changes that should be made
regardless of the number of tuples involved, it's incorrect to skip it
when memtupcount = 1; the number of cycles saved is minuscule anyway.
An alternative solution would be to pull the state changes out to the
call site in tuplesort_performsort, but keeping them near the corresponding
changes in make_bounded_heap seems marginally cleaner.  Noticed by
Greg Stark.

Modified Files:
--------------
    pgsql/src/backend/utils/sort:
        tuplesort.c (r1.77 -> r1.78)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c?r1=1.77&r2=1.78)