pgsql: Stop btree indexscans upon reaching nulls in either direction.

Stop btree indexscans upon reaching nulls in either direction.

The existing scan-direction-sensitive tests were overly complex, and
failed to stop the scan in cases where it's perfectly legitimate to do so.
Per bug #6278 from Maksym Boguk.

Back-patch to 8.3, which is as far back as the patch applies easily.
Doesn't seem worth sweating over a relatively minor performance issue in
8.2 at this late date. (But note that this was a performance regression
from 8.1 and before, so 8.2 is being left as an outlier.)

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d23165bbc306005f687756a20d9af807d665a1f2

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