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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ca918f99aaa5fe108248ea2380f637bab09a88c2

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(-)