Handle old PRs with deleted branches.

[marcdumais-work]

For a non-obvious reason, trying to review code from PR 1855, resulted in an instance where the source code was not cloned. In consequence, directory /workspace/ was empty and no workspace was set. I just tried with a different PR, and this did not happen.

[svenefftinge]

I think this happened because the branch was deleted already. We need to detect that and tell the user.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.