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

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

This reverts commit d23165bbc306005f687756a20d9af807d665a1f2.
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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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/9f19cc28a439c88c73bc6083ebd9cd603a65b7fe

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