Fix error 500 when no HEAD is available

Steps to reproduce:

1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Error:

```
NoMethodError - undefined method `sha' for nil:NilClass:
  app/helpers/projects_helper.rb:281:in `readme_cache_key'
  app/views/projects/_readme.html.haml:10:in `_app_views_projects__readme_html_haml___2036282917939462960_70154565285700'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:339:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:143:in `render'
  rack-mini-profiler (0.9.0) lib/mini_profiler/profiling_methods.rb:108:in `block in profile_method'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:306:in `render_partial'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:279:in `block in render'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:278:in `render'
  actionview (4.1.11) lib/action_view/renderer/renderer.rb:47:in `render_partial'
  actionview (4.1.11) lib/action_view/helpers/rendering_helper.rb:35:in `render'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `block in render_with_haml'
  haml (4.0.5) lib/haml/helpers.rb:89:in `non_haml'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `render_with_haml'
  app/views/projects/show.html.haml:47:in `_app_views_projects_show_html_haml__1458303859692972698_70154948164180'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
```

Closes https://github.com/gitlabhq/gitlabhq/issues/9484, but there may be some other issue there.

See merge request !1041