zuul gates all of the contained projects in an effective single
timeline. This means that OpenStack, across all of the projects, does
already have a sequence of combinations that have been explcitily
tested, but it's non-trivial to go from a single commit of a particular
project to the commits that were tested with it.
Gerrit's submodule tracking feature will update a super project every
time a subproject is updated, so the specific sequence created by zuul
will be captured by the super project commits.