Restrict commit area resize to vertical.

Otherwise, it is possible to make things unaligned, and worse: make the 70 char mark pop out of the textarea as in the screenshot:

![screenshot from 2014-08-12 20 40 18 commit new file resize vertical break](https://cloud.githubusercontent.com/assets/1429315/3895873/e1276d76-2251-11e4-92f8-5c66a36e4011.png)