Thanks to Hans-Christoph Steiner.

Thanks to Hans-Christoph Steiner for the suggestion.

At the request of Holger, we just don't create the HTML file if there is no
differences to report.

difflib is unbearably slow for any non-trivial output. Calling out to vim is
much faster and the output is still quite nice. Crazy trick, though.

We are going to examine every files in the package, so there's no
added value in displaying a diff of the md5sums files. They are
different, that's enough information.

libmagic tends to be costly when called on so many files