Improve exception handling for issue fork initial branch creation

Approximately 3% of the time, issue fork creation gets a “Exception in initial branch creation for …: 403 Forbidden”

We should handle that exception and not record metadata about the initial branch until it is created, so the UI doesn’t drift from reality. We currently record the metadata early, instead of waiting for the usual webhook, so people see it is (usually) available right away.

Additionally, we could do one retry with a sub-second delay. This smells like we’re doing something before GitLab is ready for it in the new repository. Since it succeeds much of the time, we probably only need to wait.