Hub merge should use rep name from api

See #379. Seems like an easy fix so I went ahead and did that! (even though I know hub is transitioning to go).

If you would like to try this on an actual repository, you can use it on https://github.com/ivantsepp/yolo10/pull/1 for now :wink: 

From the commit:

The repository name of the pull request head doesn't necessarily match the
name of the project's repository. This can happen when a user forks a project
but already has a repository of the same name. Use the name returned in the
api response for the fetch url to avoid this edge case.