Some runtime data that stores which sockets can be fields and which
can't is not stored in the file, but only calculated when necessary.
When opening a file, the node tree update function was called, which
recalculated this data, but that was explicily turned off for undo.
This exposes a fundamental issue with undo, the ID caching system for
undo, and how it relates to node trees in particular. Ideally this call
couldn't be necessary at all. In the future it could be removed by
adding a runtime struct to node trees, and calculating its contents
on-demand instead of preemtively.
Differential Revision: https://developer.blender.org/D12699
f18ab3470f