Create a bookmark in Chrome or Firefox and copy/paste this code into the URL.
javascript:(function()%7Bvar%20a%3Ddocument.createElement('style')%2Cb%3Bdocument.head.appendChild(a)%3Bb%3Da.sheet%3Bb.insertRule('.udiff-line%20.source%20%7B%20height%3A%20initial%3B%20white-space%3A%20pre-wrap%3B%20word-wrap%3A%20normal%3B%20%7D'%2C0)%3B%7D)()
This bookmarklet adds the following user-style: