Allow "merge style" pull requests to have just the title "Merge". If the title is just "Merge" then the bots will rewrite the title to "Merge <HEAD>" where <HEAD> is the HEAD of the pull request source branch. This is useful if the integrator knows that they want the HEAD of the pull request source branch as the second parent for the resulting merge commit (this removes the need to manually specify the HEAD hash in the PR title).