merge_into_manifest: add merge_status check before merge attempt
We had some cases where the merge job added rebases of a commit where rebasing was not necessary at all, e.g.: https://gitlab.com/garz-fricke/yocto/layers/meta-guf-machine/-/jobs/1270293595
This is obviously a known issue in GitLab concerning the HTTP return value of the merge endpoint: https://gitlab.com/gitlab-org/gitlab/-/issues/196962
Suggested workaround is to manually check merge_status and wait until it is ready before attempting to merge the MR.