Gitlab::Git::Repository#update_branch

This method was hiding somewhere in gitlab-ee/lib/gitlab/git/repository.rb. It uses OperationService to update a branch on behalf of a user. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6221/diffs

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Stages:

• ~"Client Preparation": #1250 (closed)

• RPC Design: #1251 (closed)

• Server Implementation: #1257 (closed)

• ~"Client Implementation": #1252 (closed)

• ~"Acceptance Testing": #1310 (closed)

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