pgsql-server: Change get_rule_expr so that when the input is a List, it

Log Message:
-----------
Change get_rule_expr so that when the input is a List, it displays the
list elements comma-separated instead of barfing.  This allows elimination
of half a dozen redundant copies of that behavior, and also makes the
world safe again for pg_get_expr() applied to pg_index.indexprs, per gripe
from Alexander Zhiltsov.

Modified Files:
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    pgsql-server/src/backend/utils/adt:
        ruleutils.c (r1.181 -> r1.182)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/adt/ruleutils.c.diff?r1=1.181&r2=1.182)