Proper handling of external PRs

[modulo11]

Before PR#99 rebazer crashed when trying to checkout an external PRs branch which does not exist in our origin repository. We should discuss how we want to handle this within rebazer:

• Ignore the handling of external PRs
• Don't try to rebase external PRs, but merge if conditions are met
• ...

[beatngu13]

AFAIK one gets write permissions to a PR branch from another repo, at least this was the case with e.g. the Gradle PRs in recheck.

I think it would be helpful to also handle external PRs—if it's not too much work. What was the exact error message?