Disable PR Merge

[81reap]

@lovasoa starting a discussion as this is an opinionated thing. personally, I am a fan of always rebasing and rarely squashing rather than merging. I think of mainline like a rope that's being braided commit by commit.

currently the repo allows for merging which leads to merge commits on PRs that wouldn't need them like in #1432 and #1433

[81reap]

I also find that it works better with stacked commits, I saw you were starting to use those with #1439 and #1440