Code in charge of fixing invalid liboverride hierarchy roots would be
too agressive in re-assigning new root in replacement of old one. When
the invalid initial case was a 'low-level' type of ID (like a nodetree
e.g.), it could 'contaminate' many valid data (objects etc.) and force
them into its own local hierarchy.
This commit fixes the issue by storing all 'processed as valid' IDs into
a set, and ensuring that once an ID has been put into that set, its
hierarchy root is not changed anymore.
in other words, it changes the old behavior of 'last encountered hierarchy
wins' to 'first encountered hierarchy wins'. Since higher-level types of
IDs (like collections or objects) are processed first, this is the
most logical behavior too.
bf93fb0f46