suppress_redundant_updates_trigger function.

doc/src/sgml/func.sgml

src/backend/utils/adt/Makefile

src/backend/utils/adt/trigfuncs.c

src/include/catalog/pg_proc.h

src/include/utils/builtins.h

src/test/regress/expected/triggers.out

src/test/regress/sql/triggers.sql