For repos with certain merge methods disabled (such as 'merge'), simply
hitting the GitHub `/merge` endpoint is not sufficient. Instead, we need
to check which methods are actually available, then use the appropriate
method.

With this change it will:

 1. Default to 'merge' if enabled
 2. Use 'rebase' if 'merge' is disabled and rebase is enabled
 3. Use 'squash' if 'merge' is disabled