Related to #750

Description

The Squash and Merge feature on GH is super nice for maintaining clean master history and we use it for every PR we deal with. The downside is it's a huge pain to prune local branches because Git doesn't realize we merged it. I feel like this shouldn't (?) be a problem with a GH client though, because it could query to determine whether or not the branch was squashed or even examine the commits to determine if there were new local commits vs. what was squashed. You guys know more about what'd be possible but it feels like it's doable.

This would be a fantastic time-saver feature!