I'd like to subscribe to all changes to a GitHub pull request without first having to put in a comment.
Is there anyway to do that?
On the sidebar, under Notifications, you will be able to toggle your subscription to the thread.
You can also now manage notification settings at the individual issue or pull request level. This lets you manually subscribe to any issue without commenting, or unsubscribe from issues no longer relevant to your interests:
See the blog post, "Mention @somebody. They're notified." for details.