pgsql: Tweak choose_bitmap_and() heuristics in the light of example

Log Message:
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Tweak choose_bitmap_and() heuristics in the light of example provided in bug
#2075: consider an index redundant if any of its index conditions were already
used, rather than if all of them were.  Also, make the selectivity comparison
a bit fuzzy, so that very small differences in estimated selectivities don't
skew the results.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/optimizer/path:
        indxpath.c (r1.191.2.2 -> r1.191.2.3)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.191.2.2&r2=1.191.2.3)