pgsql: Optimize the case where a btree indexscan has current and mark

Log Message:
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Optimize the case where a btree indexscan has current and mark positions
on the same index page; we can avoid data copying as well as buffer refcount
manipulations in this common case.  Makes for a small but noticeable
improvement in mergejoin speed.

Heikki Linnakangas

Modified Files:
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    pgsql/src/backend/access/nbtree:
        nbtree.c (r1.149 -> r1.150)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c.diff?r1=1.149&r2=1.150)
        nbtsearch.c (r1.105 -> r1.106)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c.diff?r1=1.105&r2=1.106)
    pgsql/src/include/access:
        nbtree.h (r1.103 -> r1.104)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/nbtree.h.diff?r1=1.103&r2=1.104)