Always compare with FETCH_HEAD in downtime_check

What does this MR do?

This MR fixes the downtime check for CI.

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

No.

Why was this MR needed?

Because of shallow clones the task would fail due to master.. not being a valid revision.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together