Add JS diff to artifacts

[emteknetnz]

Follow on from silverstripe/silverstripe-admin#1671 (comment)

When a JS diff fails, generate a new file that shows the diff between the two files that are different and copy it to the artifacts director. Will need to checkout or download the original file again.

If the diff isn't too large, also output it to the console for convenience

git diff --word-diff --word-diff-regex=[^;] is one option to generate the diff

May wish to string replace ; with ;\n before doing the diff for the file

[emteknetnz]

Probably don't need this any more, the random js diff only happens in 4.13 which is now only is security support mode. Workaround solution is to simply rebuild the bundle until the signature matches