pgsql: Actually there's a better way to do this, which is to count

Log Message:
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Actually there's a better way to do this, which is to count tuples
during the vacuumcleanup scan that we're going to do anyway.  Should
save a few cycles (one calculation per page, not per tuple) as well
as not having to depend on assumptions about heap and index being
in step.
I think this could probably be made to work for GIST too, but that
code looks messy enough that I'm disinclined to try right now.

Modified Files:
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    pgsql/src/backend/access/nbtree:
        nbtree.c (r1.139 -> r1.140)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c.diff?r1=1.139&r2=1.140)