Git bundle repository

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Stages:

• RPC Design: #928 (closed)

• ~"Client Implementation": #929 (closed)

• Server Implementation: #930 (closed)

• ~"Acceptance Testing": #973 (closed)

RPC Endpoints:

• RepositoryService::CreateBundle

Known Client Routes:

• Gitlab::ImportExport::CommandLineUtil#git_bundle

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See #944 (closed) for the importing feature.