I think the OPs concern was more with the commit history in the PR being polluted with merge commits.

I do agree that merge commits can obfuscate history somewhat. However, I agree with dimes that its better to not rewrite history just to keep the commit history clean... the cleanliness is not worth the price for inability to collaborate effectively.

Also, any professional code review tool will not let merge commits affect your review. Highly recommend reviewable.io for this.