Consider a base branch (master), a feature branch (feat). Both have been updated since their common parent, and now there is a pull request with master as the base branch, and feat as the compare branch.
If there are conflicts and you use the web editor to resolve and commit them, they will be committed into feat first. This is great! I can pull feat, test that the code works, and all is well in the world.
If there are no conflicts, then this does not happen. The merge is not committed into feat, and certainly not as an intermediate step to let you check that all is well.
I know the “proper” workflow is to do a command-line git merge master from feat before the pull request. But it would be really neat if there happened to be a way to configure GitHub to do this for you?