`gh pr merge --auto`: don't submit request if already enabled

### Describe the feature or problem you’d like to solve

`gh pr merge --auto` should check if auto-merge is already enabled for a PR. Right now, it's treating the same as if not enabled, so a request is sent regardless.

### Proposed solution

Check if auto-merge is enabled first. If it is, then print a message "✓ Auto-merge already enabled for PR #number"