python - fe: Reduce the identifier size for prepared statements.

Log Message:
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Reduce the identifier size for prepared statements.

In, at least one, 7.4 there seems to be a more significant limitation on the
size of the statement identifier string, so go ahead and cut the size on all
identifiers.
(Really, it doesn't need to be so long to begin with and it *sometimes*
causes problems by being so long, so adjust it make everyone happy without
really sacrificing anything at all.)

Modified Files:
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    fe/src:
        tracenull.py (r1.7 -> r1.8)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/fe/src/tracenull.py.diff?r1=1.7&r2=1.8)