testdriver: Wrong baseline commit for MR

I was quite happy to see that my latest perf changes to a Draft MR finally resulted in a positive change: https://gitlab.haskell.org/ghc/ghc/-/jobs/881855

But suspicious as I was, I looked through the log and found that it says

* All baseline commits are 0a5fdcfc52

and 0a5fdcfc is the 10th commit of a batch of 11 commits of !5349 (at the time of writing) and the parent of b73e70d7, the very commit I pused.

It seems that as long as I don't rebase (to change the base commit or to squash my 11 commits), I will get misleading perf numbers. And I think that if I had already documented a compiler flag I use for development, I could easily assign my MR to Marge, bogging down our CI merge train, because Marge's MR will surely flag the offending perf regessions.