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

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

This reverts commit 048fffed55ff1d6d346130e4a6b7be434e81e82c.
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_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/5cd7b682427d0e912b3ddf7f4910d52089e0df71

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