<span class="pln">$ GIT_PAGER</span><span class="pun">=</span><span class="str">''</span><span class="pln"> git diff</span>
Eight years later I find a superior answer, from http://superuser.com/questions/777617/line-wrapping-less-in-os-x-specifically-for-use-with-git-diff:
git config core.pager `fold -w 80 | less`