pgsql: Quick hack to make the VXID of a prepared transaction be -1/XID,

Log Message:
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Quick hack to make the VXID of a prepared transaction be -1/XID,
so that different prepared xacts can be told apart in the pg_locks
view. Per suggestion from Florian.

Modified Files:
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pgsql/src/backend/access/transam:
twophase.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c?r1=1.33&r2=1.34)