Removed ndiff from POSIXFile, only attempt unified diff.

This removes the ndiff, which is a bit extraneous and can cause up to 30 second
timeouts on larger files with many changes. unified_diff is faster and
generally more applicable, and is already what is shown to the admin on
interactive mode. This can save up to 30 seconds per file, per run.

A future commit can take the resulting unified diff and recreate the ndiff
specifically for the reporting if that is desired.
udiff doesn't mean unified diff, it means unicode diff
Fixed up test cases relating to _diff from POSIXFile
Fixed lint test error.
Clean up unicode diff handling a little more.