Fix Error 500 after renaming a project path

Renaming the path of a project would result in an Error 500 due to `@repository` being memoized with the old path. An Error 500 would result:

```
Started GET "/testing2/test1" for 127.0.0.1 at 2016-04-04 12:42:30 +0000
Processing by ProjectsController#show as HTML
  Parameters: {"namespace_id"=>"testing2", "id"=>"test1"}
Completed 200 OK in 637ms (Views: 194.2ms | ActiveRecord: 111.8ms)
Started GET "/testing2/test1/edit" for 127.0.0.1 at 2016-04-04 12:42:33 +0000
Processing by ProjectsController#edit as HTML
  Parameters: {"namespace_id"=>"testing2", "id"=>"test1"}
Completed 200 OK in 594ms (Views: 183.8ms | ActiveRecord: 87.4ms)
Started PATCH "/testing2/test1" for 127.0.0.1 at 2016-04-04 12:42:41 +0000
Processing by ProjectsController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"[FILTERED]", "project"=>{"nam
e"=>"test123", "path"=>"test123"}, "namespace_id"=>"testing2", "id"=>"test1"}
Completed 500 Internal Server Error in 1852ms (ActiveRecord: 124.0ms)

ActionView::Template::Error (no repository for such path):
    2:   %legend
    3:     Builds:
    4:
    5:   - unless @repository.gitlab_ci_yml
    6:     .form-group
    7:       .col-sm-offset-2.col-sm-10
    8:         %p Builds need to be configured before you can begin using Contin
uous Integration.
  app/models/repository.rb:59:in `block in empty?'
  lib/repository_cache.rb:19:in `fetch'
  app/models/repository.rb:59:in `empty?'
  app/models/repository.rb:471:in `gitlab_ci_yml'
  app/views/projects/_builds_settings.html.haml:5:in `_app_views_projects__build
s_settings_html_haml__782034335636359229_73397600'
  app/views/projects/edit.html.haml:87:in `block in _app_views_projects_edit_htm
l_haml___2388082585934859365_47390860'
  app/views/projects/edit.html.haml:8:in `_app_views_projects_edit_html_haml___2
388082585934859365_47390860'
  app/controllers/projects_controller.rb:54:in `block (2 levels) in update'
  app/controllers/projects_controller.rb:43:in `update'
  lib/gitlab/middleware/go.rb:16:in `call'
```

Closes #14885

See merge request !3528