Cache Repository#root_ref within a request

When an empty project is loaded in the UI, there are 15 separate Gitaly FindDefaultBranch calls to determine the root_ref. Previously, it was not cached even within the request. This change caches it within the request so only a single FindDefaultBranch RPC is needed.

The calls come from:

• 6 from anchor data: https://gitlab.com/gitlab-org/gitlab-ce/blob/6fc43e6f17bcb17f9754c571bf4fadd603abe82b/app/presenters/project_presenter.rb#L55-60
• 6 from can_current_user_push_to_default_branch? from the anchor data calls
• 3 from can_current_user_push_to_default_branch? in app/views/projects/empty.html.haml

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58684