pgsql: Use correct symbol for minimum int64 value

Use correct symbol for minimum int64 value

The old code used SEQ_MINVALUE to get the smallest int64 value. This
was done as a convenience to avoid having to deal with INT64_IS_BUSTED,
but that is obsolete now. Also, it is incorrect because the smallest
int64 value is actually SEQ_MINVALUE-1. Fix by writing out the constant
the long way, as it is done elsewhere in the code.

Branch
------
REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/84d67920450717d458cfaad7876fa8279fd7ced0

Modified Files
--------------
contrib/btree_gin/btree_gin.c | 10 ++--------
1 file changed, 2 insertions(+), 8 deletions(-)