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.