pgsql: Fix extraction of week and quarter fields from intervals.

Fix extraction of week and quarter fields from intervals.

"EXTRACT(WEEK FROM interval_value)" formerly threw an error.
Define it as "tm->tm_mday / 7".  (With C99 division semantics,
this gives consistent results for negative intervals.)

"EXTRACT(QUARTER FROM interval_value)" has been implemented
all along, but it formerly gave extremely strange results for
negative intervals.  Fix it so that the output for -N months
is the negative of the output for N months.

Per bug #18348 from Michael Bondarenko and subsequent discussion.

Discussion: https://postgr.es/m/18348-b097a3587dfde8a4@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6be39d77a70df52d5a0f2eb414ef9901ccf17e5a

Modified Files
--------------
doc/src/sgml/func.sgml                 | 13 +++++++-
src/backend/utils/adt/timestamp.c      | 18 +++++++++-
src/test/regress/expected/interval.out | 61 ++++++++++++++++++++++++++--------
src/test/regress/sql/interval.sql      | 18 ++++++++++
4 files changed, 94 insertions(+), 16 deletions(-)