Flatten the directory hierarchy while there is only one directory descendant

In some languages, especially java, the directory structure reflects the package structure.This means that using the file tree browser to reach the directories that matter you have to click a lot of directories containing only another subdirectory.

This merge request tries to enhance this by showing and providing the link to the first directory that doesn't contain only one directory.

See also [this feature request](http://feedback.gitlab.com/forums/176466-general/suggestions/6236769-jump-forward-to-the-first-folder-that-has-more-tha)

This is very similar to what [GitHub does](https://github.com/blog/1877-folder-jumping), but I don't think graying out the empty directories is really necessary.

Screenshots: [Before](https://gitlab.com/uploads/marmis85/gitlab-ce/b683a0000f/before.png) - [After](https://gitlab.com/uploads/marmis85/gitlab-ce/a23725a1c1/after.png)

I'm actually very new at ruby, rails, haml, and almost everything involved in Gitlab's development...
From what I could understand, when we render each directory item in the list we only have the data structure representing the item but not his descendants.
To iterate through the hierarchy I call Gitlab::Git:Tree.where() but I'm not sure if this is the most efficient way.

I also considered making every directory a clickable link, but I tried that and in the end it wasn't very useful and reduced the click target for the most useful directory.

See merge request !275