Use git config color.diff for `git diff` pretty print

[vidartf]

Git supports changing which colors to use for printing diffs through its config system, see https://git-scm.com/book/tr/v2/Customizing-Git-Git-Configuration (heading "Colors in Git") and http://shallowsky.com/blog/programming/gitcolors.html. We should consider using these in the git diff driver for pretty printing. At the very least we could check whether or not to use colors (git config colors.ui / git config colors.diff true/false/auto).

Partially related to #240.