[WIP] Make tree new file link more visible.

- I remember the first time I wanted to create a new file from the web UI searching for the button. It's harsh on new users.
- every time I want to create a new file I have to concentrate to find the little `+`.
- more coherent: GitLab uses "plus sign `+` text" green buttons for all "create new thing" links, including the analogous wiki "create new page" button that also adds a file to a Git repo.

![screenshot from 2014-05-01 15 50 05](https://cloud.githubusercontent.com/assets/1429315/2852618/e080c720-d137-11e3-98ec-334b3384d765.png)

This is just a mock up, I'll improve if people are for it.

Main difficulty: to look nice the button has to be as high as the branch dropdown to its left, but buttons are larger than dropdowns. I wish buttons, text fields and dropdowns all the same height. Analogous UI problem on the `/network` tab due to different heights of search field and dropdown.