Adds `# frozen_string_literal: true` to spec/models ruby files

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.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54862

Private commit emails were introduced in !22560, but some parts of
GitLab were not updated to take account of them. This commit adds
support in places that were missed.

Fix committer typo

Closes #51808

See merge request gitlab-org/gitlab-ce!21899

Even if it doesn’t save lines of code, since people will tend to use
code they’ve seen. And `SafeRequestStore` is safer since you
don’t have to remember to check `RequestStore.active?`.