Fix EXPLAIN output for subplans in MERGE.

Given a subplan in a MERGE query, EXPLAIN would sometimes fail to
properly display expressions involving Params referencing variables in
other parts of the plan tree.

This would affect subplans outside the topmost join plan node, for
which expansion of Params would go via the top-level ModifyTable plan
node.  The problem was that "inner_tlist" for the ModifyTable node's
deparse_namespace was set to the join node's targetlist, but
"inner_plan" was set to the ModifyTable node itself, rather than the
join node, leading to incorrect results when descending to the
referenced variable.

Fix and backpatch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCWAv-sZuH%2BwG5xJ-%2BGt7qGNGX8wUQd3XYydMFDKgRB9nw%40mail.gmail.com

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