modify guarantee_final_pipeline to run final job before merging

Fix #9 (closed)

this MR cherry-picks two commits from marge-org/marge-bot!387 (merged) and fix a few quirks related to our margebot version.

We also modify the code so that the additional pipeline is only created when the MR was not rebase by marge, to avoid duplicated pipelines. This is explained in more detail here: marge-org/marge-bot#386