Remove direct repository access from db migrations

Example: https://gitlab.com/gitlab-org/gitlab-ce/blob/bb9d360c0a7daed6aa08a0635e084c314c2c8b3e/db/migrate/20161124141322_migrate_process_commit_worker_jobs.rb

You can find them with git grep legacy_disk_path db.

It looks like we won't need any new RPC's for this but I'm not 100% sure.

Related, but can probably be done in parallel to: #1108 (closed)

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Stages:

• RPC Design: Create Issue

• ~"Client Implementation": Create Issue

• Server Implementation: Create Issue

• ~"Acceptance Testing": Create Issue

RPC Endpoints:

• Endpoint::Name

Known Client Routes:

• Known client endpoints

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