Rebase MR still manage to slowdown Gitlab from fetching

Our mitigations to lower the fetching overhead from CI pipelines did not consider rebase merge requests.

Today we saw that a pipeline triggered by !1350 (merged) caused a massive slow down of our Gitlab instance.

Since we only fetch 500 commits starting from the HEAD of this repository default branch and between firefox releases there can be around 3000-4000 commits, a job in a rebase MR pipeline will fetch that amount and likely more per job -- not good.

We are lucky that MR has all pipelines failing for an unrelated reason, heh.