Support file diffs

[StanleyGoldman]

Initially requested here: github-beta/unity-preview#39

Feature Request: Add the ability to quickly show change diffs through the UI. Often this is done using 'Ctrl-D' or double-clicking a changed file.

[thorade]

Would be nice to also have diffs for images, as done in the github web UI:
https://help.github.com/articles/rendering-and-diffing-images/