Alert user if there are unpushed commits on the branch they are merging. #969
Description
This was brought up by @vilmibm in #899 (review)
I fixed the merge conflict locally and instinctively ran gh pr merge again, still getting the merge conflict error because I hadn't pushed. A future follow-up might be to notice that 1) the PR is question is the "current" PR and 2) there are unpushed commits and then reporting that to the user ("! Unpushed commits for pull request 123")