Refactor GitAccess to use instance variables.

app/controllers/projects/merge_requests_controller.rb

app/helpers/branches_helper.rb

app/helpers/tree_helper.rb

app/services/files/create_service.rb

app/services/files/delete_service.rb

app/services/files/update_service.rb

lib/api/internal.rb

lib/api/merge_requests.rb

lib/gitlab/backend/grack_auth.rb

lib/gitlab/git_access.rb

lib/gitlab/git_access_wiki.rb

spec/lib/gitlab/git_access_spec.rb

spec/lib/gitlab/git_access_wiki_spec.rb