pgsql: Fix pg_stat_statements for MERGE

Fix pg_stat_statements for MERGE

We weren't jumbling the merge action list, so wildly different commands
would be considered to use the same query ID.  Add that, mention it in
the docs, and some test lines.

Backpatch to 15.

Author: Tatsu <bt22nakamorit@oss.nttdata.com>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/d87e391694db75a038abc3b2597828e8@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/249b0409b181311bb1c375311e43eb767b5c3bdd

Modified Files
--------------
.../expected/pg_stat_statements.out                | 41 +++++++++++++++++++++-
.../pg_stat_statements/sql/pg_stat_statements.sql  | 22 ++++++++++++
doc/src/sgml/pgstatstatements.sgml                 |  4 +--
src/backend/nodes/nodeFuncs.c                      |  4 +--
src/backend/utils/misc/queryjumble.c               | 11 ++++++
5 files changed, 77 insertions(+), 5 deletions(-)