pgsql: Fix best_inner_indexscan to actually enforce that an "inner

Log Message:
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Fix best_inner_indexscan to actually enforce that an "inner indexscan" use
at least one join condition as an indexqual.  Before bitmap indexscans, this
oversight didn't really cost much except for redundantly considering the
same join paths twice; but as of 8.1 it could result in silly bitmap scans
that would do the same BitmapOr twice and then BitmapAnd these together :-(

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/optimizer/path:
        indxpath.c (r1.191.2.5 -> r1.191.2.6)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.191.2.5&r2=1.191.2.6)