EE Repository#upstream_branches and repository mirroring in general

Repository#upstream_branches is an EE method that calls into Gitlab::Git::RepositoryMirroring and does bad stuff.

We should also take a closer look at repository mirroring in general. There are suspicious-looking methods in https://gitlab.com/gitlab-org/gitlab-ee/blob/e07277085da57fa09bf4091a565a85fbc9141c27/lib/gitlab/git/repository_mirroring.rb and we need to be sure they only get called from gitaly-ruby, if at all.

Added 20180709: Get rid of test for remote_branches in spec/models/repository_spec.rb; remote_branches is local to gitaly-ruby. Should not be tested in the gitlab-ce/ee test suite.

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Stages:

• RPC Design: gitaly-proto!202 (diffs)

• Server Implementation: !799 (merged)

• ~"Client Implementation": https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6583

• ~"Acceptance Testing": #1312 (closed)

RPC Endpoints:

• Endpoint::Name

Known Client Routes:

• Known client endpoints

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