pgsql: Fix contrib/btree_gist to handle collations properly.

Fix contrib/btree_gist to handle collations properly.

Make use of the collation attached to the index column, instead of
hard-wiring DEFAULT_COLLATION_OID.  (Note: in theory this could require
reindexing btree_gist indexes on textual columns, but I rather doubt anyone
has one with a non-default declared collation as yet.)

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/bb850306307d3d6ebb611c4039ae127236eb1699

Modified Files
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contrib/btree_gist/btree_bit.c       |   57 ++++++++-----
contrib/btree_gist/btree_bytea.c     |   53 ++++++++----
contrib/btree_gist/btree_numeric.c   |   52 +++++++----
contrib/btree_gist/btree_text.c      |   43 +++++----
contrib/btree_gist/btree_utils_num.c |    8 +-
contrib/btree_gist/btree_utils_var.c |  155 ++++++++++++++++------------------
contrib/btree_gist/btree_utils_var.h |   38 ++++-----
7 files changed, 219 insertions(+), 187 deletions(-)