pgsql: Tweak trace_sort code to show the merge order (number of active

Log Message:
-----------
Tweak trace_sort code to show the merge order (number of active input
tapes) for each merge step.  This will give us some idea of how effective
the merge distribution algorithm is.

Modified Files:
--------------
    pgsql/src/backend/utils/sort:
        tuplesort.c (r1.63 -> r1.64)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c.diff?r1=1.63&r2=1.64)