pgsql: Fix loss of fractional digits for large values in cash_numeric()

Fix loss of fractional digits for large values in cash_numeric().

Money values exceeding about 18 digits (depending on lc_monetary)
could be inaccurately converted to numeric, due to select_div_scale()
deciding it didn't need to compute any fractional digits. Force
its hand by setting the dscale of one division input to equal the
number of fractional digits we need.

In passing, rearrange the logic to not do useless work in locales
where money values are considered integral.

Per bug #15925 from Slawomir Chodnicki. Back-patch to all supported
branches.

Discussion: https://postgr.es/m/15925-da9953e2674bb5c8@postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/01e0538e8b12591283835a2ff629428262684574

Modified Files
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src/backend/utils/adt/cash.c | 56 ++++++++++++++++++++++++-------------
src/test/regress/expected/money.out | 16 ++++++++++-
src/test/regress/sql/money.sql | 6 +++-
3 files changed, 57 insertions(+), 21 deletions(-)