pgsql: Fix make_timestamp[tz] to accept negative years as meaning BC.

Fix make_timestamp[tz] to accept negative years as meaning BC.

Previously we threw an error.  But make_date already allowed the case,
so it is inconsistent as well as unhelpful for make_timestamp not to.

Both functions continue to reject year zero.

Code and test fixes by Peter Eisentraut, doc changes by me

Discussion: https://postgr.es/m/13c0992c-f15a-a0ca-d839-91d3efd965d9@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a094c8ff53523e88ff9dd28ad467618039e27b58

Modified Files
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doc/src/sgml/func.sgml                  | 12 ++++++++++--
src/backend/utils/adt/timestamp.c       | 14 +++++++++-----
src/test/regress/expected/date.out      |  2 ++
src/test/regress/expected/timestamp.out | 11 ++++++++++-
src/test/regress/sql/date.sql           |  1 +
src/test/regress/sql/timestamp.sql      |  5 ++++-
6 files changed, 36 insertions(+), 9 deletions(-)