Bors: 422 Update is not a fast forward

[Mark-Simulacrum]

Periodically we'll see bors not manage to push the built PR into master for some reason.

This is possibly because of submodules not being synchronized? Wild stab in the dark.

[kennytm]

cc servo/homu#24.

Could bors (homu) automatically wait for 5 seconds, and then push again in case of 422?

[Eh2406]

If we can track down where in the code this come from, I will try and add a retry.

[kennytm]

@Eh2406

https://github.com/servo/homu/blob/4424aedb08763c76e1ac682c96fea0adbd7232ff/homu/server.py#L518-L524

[Eh2406]

Is the error that is being cout there thrown by github_set_ref? If so can we add a retry in github_set_ref, as it already has 422 special cases?

[Eh2406]

Move to close?