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

Because of the way ExtractPaths works, `params[:format]` wouldn't
necessarily be available at the time this filter was running, and so it
would erroneously redirect to `new_user_session_path`

Takes tree-ish + path as ID