Support stacking PRs across repositories

[starboardtolkien]

Plenty of projects are split across multiple repos, for example separate frontend/backend, shared libraries, microservices, schema/proto repos, and so on. A single feature frequently needs PRs across several repos.

[skarim]

This is an interesting concept we have discussed, but it is outside the scope of stacked PRs, which are limited to a single git repo. Coordinating merges across separate repos would likely involved a completely separate concept — one that is less "stacked" and more akin to "groups."