pgsql: Fix corner-case bug introduced with HOT: if REINDEX TABLE

Log Message:
-----------
Fix corner-case bug introduced with HOT: if REINDEX TABLE pg_class (or a
REINDEX DATABASE including same) is done before a session has done any other
update on pg_class, the pg_class relcache entry was left with an incorrect
setting of rd_indexattr, because the indexed-attributes set would be first
demanded at a time when we'd forced a partial list of indexes into the
pg_class entry, and it would remain cached after that. This could result
in incorrect decisions about HOT-update safety later in the same session.
In practice, since only pg_class_relname_nsp_index would be missed out,
only ALTER TABLE RENAME and ALTER TABLE SET SCHEMA could trigger a problem.
Per report and test case from Ondrej Jirman.

Modified Files:
--------------
pgsql/src/backend/catalog:
index.c (r1.300 -> r1.301)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c?r1=1.300&r2=1.301)
pgsql/src/backend/utils/cache:
relcache.c (r1.272 -> r1.273)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c?r1=1.272&r2=1.273)