We still had cases where a merge request was rebased multiple times after an
initial (actually required) rebase:

https://gitlab.com/garz-fricke/yocto/infrastructure/ci-test/minimal-bar/-/jobs/1345964713

I assume that GitLab simply does not update the MR that fast and so we attempt
to do a rebase, while the MR still reports it is merge-ready.

Adding a wait loop now where the MR's sha is compared to the one of the new
commit in order to make sure that the MR has been updated before we attempt
another rebase.