The GitHub Visual Studio extension v2.4.3.1737 just did this and it's pure genius. See https://haacked.com/archive/2018/03/15/github-vs-pr-features/ for details. This would probably be easier to add this feature than setting up the UI and persistence to implement the idea at https://devart.uservoice.com/forums/145340-review-assistant/suggestions/5744139-option-to-set-a-default-action-when-opening-compar and nets the same result.