pgsql: Reject MERGE in CTEs and COPY

Reject MERGE in CTEs and COPY

The grammar added for MERGE inadvertently made it accepted syntax in
places that were not prepared to deal with it -- namely COPY and inside
CTEs, but invoking these things with MERGE currently causes assertion
failures or weird misbehavior in non-assertion builds. Protect those
places by checking for it explicitly until somebody decides to implement
it.

Reported-by: Alexey Borzov <borz_off(at)cs(dot)msu(dot)su>
Discussion: https://postgr.es/m/17579-82482cd7b267b862@postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/92af9143f13df8c54362ebbd4397cb53f207ff2d

Modified Files
--------------
src/backend/commands/copy.c | 6 ++++++
src/backend/parser/parse_cte.c | 7 +++++++
src/test/regress/expected/merge.out | 14 ++++++++++++++
src/test/regress/sql/merge.sql | 10 ++++++++++
4 files changed, 37 insertions(+)