pgsql: Dept of second thoughts: after studying index_getnext() a bit

Log Message:
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Dept of second thoughts: after studying index_getnext() a bit more I realize
that it can scribble on scan->xs_ctup.t_self while following HOT chains,
so we can't rely on that to stay valid between hashgettuple() calls.
Introduce a private variable in HashScanOpaque, instead.

Modified Files:
--------------
    pgsql/src/backend/access/hash:
        hash.c (r1.114 -> r1.115)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hash.c?r1=1.114&r2=1.115)
        hashsearch.c (r1.57 -> r1.58)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashsearch.c?r1=1.57&r2=1.58)
    pgsql/src/include/access:
        hash.h (r1.94 -> r1.95)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/hash.h?r1=1.94&r2=1.95)