pgsql: Make EXPLAIN MERGE output format more compact

Make EXPLAIN MERGE output format more compact

We can use a single line to print all tuple counts that MERGE processed,
for conciseness, and elide those that are zeroes.  Non-text formats
report all numbers, as is typical.

Per comment from Justin Pryzby <pryzby@telsasoft.com>

Discussion: https://postgr.es/m/20220511163350.GL19626@telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/598ac10be1c20961baac44db773eb826f788fdfa

Modified Files
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src/backend/commands/explain.c      | 25 +++++++++++++++++++++----
src/test/regress/expected/merge.out | 35 ++++++++++-------------------------
2 files changed, 31 insertions(+), 29 deletions(-)