pgsql: Fix to_char(), to_date(), and to_timestamp() to handle negative/

Fix to_char(), to_date(), and to_timestamp() to handle negative/BC
century specifications just like positive/AD centuries.  Previously the
behavior was either wrong or inconsistent with positive/AD handling.

Centuries without years now always assume the first year of the century,
which is now documented.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ac78c4178bec7b252d22a39515ad6efd6c1a7d7e

Modified Files
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doc/src/sgml/func.sgml                    |    4 ++-
src/backend/utils/adt/formatting.c        |   49 +++++++++++++++++++----------
src/test/regress/expected/timestamp.out   |    4 +-
src/test/regress/expected/timestamptz.out |    4 +-
4 files changed, 39 insertions(+), 22 deletions(-)