preserve raw diff output

[tolry]

When generating a diff object, there is no way to retrieve the raw diff output from git. This can be useful, since lots of tools know how to process (unified) diff strings.

In my use case, I wanted to display a diff in HTML easily/quickly by using https://github.com/rtfpessoa/diff2html

If you like the idea I could make a PR.

[GenieTim]

As far as I understand, this is a requested feature, e.g. in #113 . In case you have still interest respectively still use this bundle, I will be glad to review a PR of yours