Kill diff support

[lydell]

In #1071 I requested diff support in the HTML reporter, but it was closed pointing to http://unexpected.js.org/.

In other words, the response was “it’s up to the assertion library”.

At first I thought that was a shame, because it felt like such duplicated effort for each assertion library to implement diffing (of course, every assertion library doesn’t have to do it; some might dislike diffing and consider it a feature, but most, I think, like diffs).

Now I’ve tried Unexpected (see link above) a bit, and boy do I like it! It’s diffs are pure awesomeness! When playing with it I realized that if assertion libraries do diffs themselves they can do it a lot better, since they have access to much more information than mocha. Another example is https://github.com/power-assert-js/power-assert.

mocha’s diffing has been plagued with issues and regressions, and has a bunch of open (and semi-open) issues right now: #1241, #1348, #1624, #1712.

I’d like to open the discussion on removing diffs from mocha entirely, and let it focus on doing what it does well. Leave diffs to the assertion libs which have the possibility to do it better. Just my 2¢.