Fix diff-refine-* colors for highlighting Magit hunks.

Hi!
I prefer the highlighting of hunks for Magit Mode, i.e.,
```lisp
;; Show fine (word-granularity) differences within diff hunks
(setq magit-diff-refine-hunk 'all)
```
But the result of the overlapping of `magit-diff-*` and `diff-refine-*` colors is insufficient. The proposed fix should improve the highlighting. There are two screenshots showing the difference.

Old highlighting:
![magit-diff-old](https://cloud.githubusercontent.com/assets/2260430/4444123/3baaabc0-47ee-11e4-82fb-4af548a7691e.png)
New highlighting:
![magit-diff-new](https://cloud.githubusercontent.com/assets/2260430/4444131/5c8b2b58-47ee-11e4-8c47-96c82f86d131.png)

Best regards,
Vladimir.