Add support for magit next

This commit adds support for the forthcoming [next branch][] of magit, which should be
merged in master in the near future.
magit-next defined new faces for it's diff hunks that do not play well
with the solarized dark, and this commit intend to fix it.

[next branch]: https://github.com/magit/magit/tree/next "magit-next branch on github "