pgsql: Revert "Stop btree indexscans upon reaching nulls in either dire

Revert "Stop btree indexscans upon reaching nulls in either direction."

This reverts commit 7357f92a3ef1faff0ce85f55f3c5a7f4dc820eaa.
As pointed out by Naoya Anzai, we need to do more work to make that
idea handle end-of-index cases, and it is looking like too much risk
for a back-patch.  So bug #6278 is only going to be fixed in HEAD.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/42f77244e75a7f82bd87ccf72bdd9d2eb2005f69

Modified Files
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src/backend/access/nbtree/nbtutils.c |  107 ++++++++++++++++++++-------------
1 files changed, 65 insertions(+), 42 deletions(-)