Support diff syntax

[ernstwi]

I had to look this up, so: DiffAdd, DiffDelete, etc are used for diff mode, eg when starting vim with vimdiff. The diff syntax is what's used for output of the diff program and for git diffs, and this syntax uses the similarly named highlight groups diffAdded, diffRemoved, etc.

So with this PR we get proper coloring eg when doing git commit --verbose. I could have linked diffAdded to Comment but I liked the green you use for StatusLine which pops more.

[ernstwi]

There are more groups defined in the diff syntax file, but these are the most important I figure.