Loads GitHub PRs twice when detaching

[danvk]

This was a bug introduced in #243. The parent process fetches the PR, then the child process does the same. Ideally only one would do it. The parent process might be better so that you can get error handling on the main thread. But then how do you share state?

$ webdiff https://github.com/danvk/webdiff/pull/243/files
Loading pull request danvk/webdiff#243 from GitHub...
Serving diffs on http://localhost:50467
Close the browser tab when you're done.
$ Loading pull request danvk/webdiff#243 from GitHub...