Compare views and cleanup

- Remove compare/_head and just use commits/_head since they're identical
- Add index view; extract the form into a partial
Fix various links
Remove commit_tab_class helper
Fix the remaining actions in RefsController
Use current ref for Files and Commits if available

Closes #1452
Fix paths in commits/_head
Add CommitsController spec to make sure atom feeds work
Fix logs not showing in Tree for the root path
Fix Repository role spec
Fix GFM request spec
Clean up project access spec
Remove atom Dashboard spec that no longer applies
Rename branches and tags Repo methods to branch_names and tag_names
Add branches and tags Repo methods

Simplifies the actions in RepositoriesController
Speed up request specs a bit
Speed up Compare feature step
Use Rails.root.join where appropriate
Add current_action? helper
Add nav_link helper to TabHelper
Replace various "active tab" checks with nav_link

Also remove now-unused tab_class helper