Improve headers for diff and patch of a commit

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Why are we doing this work

CommitController#show can be rendered as diff or path:

Currently, the cache headers are:

cache-control: no-cache
pragma: no-cache

However, at least the public content can be served via CDN:

cache-control: max-age=3600, public, must-revalidate, stale-while-revalidate=60, stale-if-error=300, s-maxage=60
etag: "e12a66ade493301905400bc87638b510"

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Edited by 🤖 GitLab Bot 🤖