[UpdateTestChecks] Fix update_mca_test_checks.py slowness issue

The script was using Python's difflib module to calculate the number of
lines changed so that it could report it in its status output.  It turns
out this can be very very slow on large sets of lines (Python bug 6931).
It's not worth the cost, so just remove the usage of difflib entirely.

llvm-svn: 330419