Expire related caches after changing HEAD

Originally opened at !3919 (closed) by @pmq20.

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What does this MR do?

Ensures that, for example, the README of the project is updated when its HEAD changed via the settings page.

Are there points in the code the reviewer needs to double check?

Check if I missed other similar scenarios where similar problems exist.

Why was this MR needed?

Because all cache involving HEAD should be updated when HEAD changes.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together