Add back Rugged support for retrieving a commit tree entry
This brings back some of the changes in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20176/diffs. We discovered another N+1 that hits Gitaly `TreeEntry` via the `RelativeLinkFilter`: https://gitlab.com/gitlab-org/gitlab-ce/issues/58657. When a blob is loaded with many relative links, `TreeEntry` is called for each link to scan the URI type. There are multiple paths that hit Gitaly `TreeEntry`, and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25706 did not cover all cases. This commit covers another common use case. 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_commit_tree_entry` to activate the Rugged method.
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