pgsql: Repair old performance bug in tuplesort.c/logtape.c.

Log Message:
-----------
Repair old performance bug in tuplesort.c/logtape.c. In the case where
we are doing the final merge pass on-the-fly, and not writing the data
back onto a 'tape', the number of free blocks in the tape set will become
large, leading to a lot of time wasted in ltsReleaseBlock(). There is
really no need to track the free blocks anymore in this state, so add a
simple shutoff switch. Per report from Stefan Kaltenbrunner.

Modified Files:
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pgsql/src/backend/utils/sort:
logtape.c (r1.19 -> r1.20)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/logtape.c.diff?r1=1.19&r2=1.20)
tuplesort.c (r1.62 -> r1.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c.diff?r1=1.62&r2=1.63)
pgsql/src/include/utils:
logtape.h (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/logtape.h.diff?r1=1.14&r2=1.15)