pgsql: Modify tidbitmap.c to avoid creating a hash table until there is

Log Message:
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Modify tidbitmap.c to avoid creating a hash table until there is more
than one heap page represented in the bitmap.  This is a bit ugly but
it cuts overhead fairly effectively in simple join cases.  Per example
from Sergey Koposov.

Modified Files:
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    pgsql/src/backend/nodes:
        tidbitmap.c (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/tidbitmap.c.diff?r1=1.2&r2=1.3)