pgsql: Message style improvements

Message style improvements

An improvement pass over the new stats import functionality.

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/663eecb5b6166b1a41276819affa38ba27951b5e

Modified Files
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src/backend/statistics/attribute_stats.c   | 25 +++++++------
src/backend/statistics/relation_stats.c    |  2 +-
src/backend/statistics/stat_utils.c        | 14 +++----
src/test/regress/expected/stats_import.out | 60 +++++++++++++++---------------
4 files changed, 52 insertions(+), 49 deletions(-)