pgsql: Use a bitmapset instead of a list for duplicate-column checking

Log Message:
-----------
Use a bitmapset instead of a list for duplicate-column checking in
checkInsertTargets(). Avoids O(N^2) behavior on wide target lists.

Modified Files:
--------------
pgsql/src/backend/parser:
parse_target.c (r1.129 -> r1.130)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_target.c.diff?r1=1.129&r2=1.130)