pgsql: Fix bug in PreCommit_CheckForSerializationFailure. A transaction

Fix bug in PreCommit_CheckForSerializationFailure. A transaction that has
already been marked as PREPARED cannot be killed. Kill the current
transaction instead.

One of the prepared_xacts regression tests actually hits this bug. I
removed the anomaly from the duplicate-gids test so that it fails in the
intended way, and added a new test to check serialization failures with
a prepared transaction.

Dan Ports

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1eea8e8a06bc718836c34b8f7da9420b38fc4851

Modified Files
--------------
src/backend/storage/lmgr/predicate.c         |   15 ++++++++
src/test/regress/expected/prepared_xacts.out |   47 ++++++++++++++++++++++---
src/test/regress/sql/prepared_xacts.sql      |   22 +++++++++++-
3 files changed, 77 insertions(+), 7 deletions(-)