Bring back Rugged implementation of ListCommitsByOid
This brings back changes in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20432. For users using Gitaly on top of NFS, accessing the Git data directly via Rugged may be faster than going through than Gitaly. This merge request introduces the feature flag `rugged_list_commits_by_oid` to activate the Rugged method. For one customer, we saw that ListCommitsByOid was the second highest used endpoint that may be causing increased load.
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