Add script to detect "naughty" commit histories

I originally wanted this script to be more generic and look for *any* modified binary files that are over a certain threshold, but it took ~88 minutes to run over the entire library, and almost all that time was spent in `git` itself generating commit diffs (since they're not stored on-disk in such a way that those commit diffs are inexpensive to calculate).

I also did a rough prototype in Go using https://github.com/src-d/go-git but it had essentially the same speed (as expected, since it's the underlying data that makes it expensive to calculate, not the language being used to request the data).

So, instead this looks for modified files which match `**.tar**`, which should match the ones we typically worry about anyhow (there are some other problematic cases, but they aren't going to be rebased anytime soon because they've been problematic for so long so aren't worth bending over backwards to detect).

Author: tianon