Update contrib/hstore for new GIN extractQuery API.

In particular, make hstore @> '' succeed for all hstores, likewise
hstore ?& '{}'.  Previously the results were inconsistent and could
depend on whether you were using a GiST index, GIN index, or seqscan.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=ba398969cd1b32c550345bcf6b713deeb0a7d1de

Modified Files
--------------
contrib/hstore/expected/hstore.out |    2 +-
contrib/hstore/hstore_gin.c        |  112 +++++++++++++++++++++---------------
contrib/hstore/hstore_op.c         |   14 +++--
3 files changed, 75 insertions(+), 53 deletions(-)

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Next:	From: Tom Lane	Date: 2011-01-09 23:04:37
	Subject: pgsql: Update contrib/pg_trgm for new GIN extractQuery API.
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	Subject: pgsql: Improve comment.