Include diffs to previous run in test results

[diresi]

Wouldn't it be sweet to have the test result diffs posted with the rest of the test stats?
This way I wouldn't have to look previous runs up in mattermost's history ... a big win for me at least :)

Now I'm totally unfamiliar with Jenkins internals, but it seems to me that AbstractTestResultAction provides ways to access previous runs or even diffs directly (getFailureDiffString()?).

IMO that would be a great improvement!