Cross-repo bisect: expose or push synthetic repositories

[xmo-odoo]

Thought I already had a task for this but couldn't find it so here goes: one thing that's really annoying when bisecting or bugdiving Odoo is it's hard to know what commits go together. With access to the runbot backend it's possible to check the staging heads but there's no easy way to feed that to bisect so you need to bisect by hand, and for large stagings the ganularity is low.

What should be achievable is to reify each staging / batch as a commit in a project-level repo, where each actual repo within the project is a submodule. This would have to be chained through the stagings (and batches) and thus somewhat expensive to compute, but afterwards it's just a matter of building the relevant commit(s) on the previous successful staging during staging.