pgsql: Forward-port regression test for bug #10587 into 9.3 and HEAD.

Forward-port regression test for bug #10587 into 9.3 and HEAD.

Although this bug is already fixed in post-9.2 branches, the case
triggering it is quite different from what was under consideration
at the time.  It seems worth memorializing this example in HEAD
just to make sure it doesn't get broken again in future.

Extracted from commit 187ae17300776f48b2bd9d0737923b1bf70f606e.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/04b598b230953c9b1495b1efdce0bad5cbd28eb0

Modified Files
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src/test/regress/expected/join.out |   71 ++++++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql      |   58 +++++++++++++++++++++++++++++
2 files changed, 129 insertions(+)