CI: New predefined variable: CI_MERGE_REQUEST_STATUS = open/merged/closed

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Problem to solve

I'd like to know if the MR the job is running for is open, merged or closed.

CI_MERGE_REQUEST_STATUS = open / merged / closed (or something like this)

Rationale: Some jobs are not intended for retry when the corresponding MR was closed.

Intended users

DevOps

Permissions and Security

Documentation

What does success look like, and how can we measure that?

Links / references

gitlab-ce~4107616 @ayufan @grzesiek @markglenfletcher @brendan