pgsql: Code cleanup for assign_XactIsoLevel.

Code cleanup for assign_XactIsoLevel.

The new coding avoids a spurious debug message when a transaction
that has changed the isolation level has been rolled back.  It also
allows the property to be freely changed to the current value within
a subtransaction.

Kevin Grittner, with one small change by me.

Branch
------
master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=fb4c5d2798730f60b102d775f22fb53c26a6445d

Modified Files
--------------
src/backend/commands/variable.c |   30 +++++++++++++++---------------
src/backend/utils/misc/guc.c    |    2 +-
src/include/utils/guc.h         |    1 +
3 files changed, 17 insertions(+), 16 deletions(-)