Reading gitattributes may be broken when hashed storage is in use

From: https://gitlab.com/gitlab-com/infrastructure/issues/2821#note_47684308

In %10.0 we introduced "hashed storage". When enabled, rather than storing projects on disk in a location determined by project.full_path, they are now stored in @hashed/aa/bb/full-hash. The hash is generated by SHAing the project ID.

The use of full_path needs to be re-evaluated and tested.

# Reading gitattributes broken in some cases? 
lib/gitlab/git/repository.rb:73:        @attributes = Gitlab::Git::Attributes.new(path)
lib/gitlab/git/attributes.rb:36:        full_path = File.join(@path, path)
lib/gitlab/git/attributes.rb:39:          return attrs if File.fnmatch?(pattern, full_path)
lib/gitlab/git/attributes.rb:101:        full_path = File.join(@path, 'info/attributes')
lib/gitlab/git/attributes.rb:103:        return unless File.exist?(full_path)
lib/gitlab/git/attributes.rb:105:        File.open(full_path, 'r') do |handle|