Enable rebase merge

[tustvold]

This will allow preserving PR commit history, which in certain situations (e.g. #2081) can be desirable.

The alternative would be to perform the rebase merge locally and push to master, but this feels less hacky

[alamb]

I recommend we enable this for the period of time needed to preserve the history from object_store and then we can disable it. I think the current system of squash merges in arrow-rs main works well

[tustvold]

I'll try doing the merge locally first, and we can merge this if necessary. I think I can push master, but it somewhat opaque what the asf auto-sync is actually setting