Refactor GitLab::Git rake tasks

Rake tasks include:

• Prune
• GC
• Repack

Prune is missing, although we might want to discuss to what extend we need that task.

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Stages:

• RPC Design: Create Issue

• ~"Client Implementation": Create Issue

• Server Implementation: Create Issue

• ~"Acceptance Testing": Create Issue

RPC Endpoints:

• Repository::Prune

Known Client Routes:

• rake gitlab:git:prune (https://gitlab.com/gitlab-org/gitlab-ce/blob/6b0f594c00d09a875e3cf8c830ae8cfffaa97b6c/lib/tasks/gitlab/git.rake#L3)

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/cc @jacobvosmaer-gitlab