Now it's consistent across layouts where their main menus are.

Also remove now-unused tab_class helper

Simplifies the actions in RepositoriesController

Closes #1452

- Remove compare/_head and just use commits/_head since they're identical
- Add index view; extract the form into a partial

Create just redirects to our specially-formatted #show action

This group is better for features that only deal with one project.

The main nav used active, the sub nav used current. This normalizes it.

Speeds things up a bit

Simplifies some of the "active tab" checks we're doing