Disable autojunking when possible

Starting with python 2.3a, SequenceMatcher performs automated junk
detection on its input. This "feature" can cause unexpected behavior
in some circumstances (see http://bugs.python.org/issue2986). In Python
2.7.1, a flag was added to disable automatic junk detection. This
patch disables junk detection when possible to ensure an accurate diff,
at the expense of speed. Note that there is no way to turn off junk
detection for Python versions between 2.3a and 2.7.