pgsql: Fix loss of fractional digits for large values incash_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
------
master

Details
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https://git.postgresql.org/pg/commitdiff/b9d2c5c7ac800bf20ea6cd4c556b6b3305863a8e

Modified Files
--------------
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(-)