pgsql: Tweak writetup_heap/readtup_heap to avoid storing the tuple

Log Message:
-----------
Tweak writetup_heap/readtup_heap to avoid storing the tuple identity
and transaction visibility fields of tuples being sorted.  These are
always uninteresting in a tuple being sorted (if the fields were actually
selected, they'd have been pulled out into user columns beforehand).
This saves about 24 bytes per row being sorted, which is a useful savings
for any but the widest of sort rows.  Per recent discussion.

Modified Files:
--------------
    pgsql/src/backend/utils/sort:
        tuplesort.c (r1.65 -> r1.66)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c.diff?r1=1.65&r2=1.66)
    pgsql/src/include/utils:
        tuplesort.h (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/tuplesort.h.diff?r1=1.19&r2=1.20)