ci-fairy: check (and ignore) merged MRs when checking merged requests

For the pipeline run after merging a branch, the MR itself is already in state "merged". No point in checking things on the MR itself then (at least not with the current checks), so just print a message and exit happily.

Since we expect a project to have a large number of merged requests we sort those by updated_at, in the hope that our MR is somewhere at the top of the list.

Fixes #11 (closed)