Use spr and bors together

[Kinrany]

SPR targets a temporary branch instead of the parent. This avoids the need for rebases but makes it impossible to merge the PR directly. The PR simply does not have the information. This makes SPR incompatible with merge bots like bors.

I have no useful ideas of my own to offer, but for prior art, Graphite supports both stacks and merge queues. Not FOSS of course.