Clean up a repository by applying a BFG object map to it

Gitaly component of https://gitlab.com/gitlab-org/gitlab-ce/issues/19376

gitlab-rails doesn't have bulk access to internal references such a refs/keep-around. To help people reduce their repository size after running BFG and force-pushing the result to a GitLab server, we need Gitaly to process the object map file generated by BFG, and remove or update any internal references to the rewritten commits.

In this first iteration, we're just looking to remove the references entirely. In a later iteration, once GitLab is updated to handle the case gracefully, this may be changed to rewrite the references instead.