fix: magit diff view clash with .gitconfig

When a user's .gitconfig has color.diff=always (and possibly "true"), magit's diff view (i.e: TAB/d in magit-status buffer) shows plain shell color codes.
color.diff=always is a perfectly reasonable option, magit should not break in its presence.

The same effect as this change can be had by adding this to your .emacs:
(dolist (additional-options '("-c" "color.diff=false"))
  (add-to-list 'magit-git-standard-options additional-options t))