Some GitHub API requests fail with 404

[pietroalbini]

Sometimes the GitHub API returns a 404 while trying to fetch the PR's diff, right after the webhook is received. Retrying those requests later return the correct data. This is probably some eventual consistency on the GitHub side.

We should retry that request after 1 second if it fails with a 404, to prevent highfive crashing.

[dwijnand]

Perhaps this is why sometimes highfive doesn't assign anyone to PRs in the cargo repo.