Allow any non-split work resume

[zanerock]

Overview

When resuming work, we currently require everything be on the main branch in order to avoid split work (where some projects are on the work branch and others are not). However, we could be a bit more flexible if we analyzed the work units implied by every project in the resuming unit and if all the projects in the current work unit of a project would be switched by the resume, then we can safely resume.

Deliverables

• add allowAnyNonSplit option and implement above.