Re: support for MERGE

Thanks everyone for the feedback.  I attach a version with the fixes
that were submitted, as well as some additional changes:

- I removed the restriction for tables inheritance and added the sample
  I showed to regression.

- I added DO NOTHING support to the WHERE MATCHED case; it previously
  only covered WHERE NOT MATCHED.

I was thinking earlier that it may be possible to clean up the
parse_merge.c code by using another RangeTblRef to process the data
source RTE.  Haven't tried yet.

This stuff is all in
https://github.com/alvherre/postgres/commits/merge-15

-- 
Álvaro Herrera           39°49'30"S 73°17'W  —  https://www.EnterpriseDB.com/
"Nunca confiaré en un traidor.  Ni siquiera si el traidor lo he creado yo"
(Barón Vladimir Harkonnen)