issue_13422: Ignore MPTT fields in serialisation if being used by staged changes

[minitriga]

Fixes: #13422

Implement a check to disable the removal of MPTT fields during object serialisation.

[github-actions]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed automatically if no further action is taken.

[arthanson]

@minitriga can you please fix the merge conflicts

[jeremystretch]

@minitriga thanks for working on this. I think the MPTT-specific concerns you raised in #13422 are symptomatic of a moderately larger issue, which is the incomplete serialization of object data. I've opened #16290 to resolve this in a more general manner, and designated it as blocking for #13422.

I'm going to work on a PR for #16290, which will effectively incorporate your changes to serialize_object() as well as some additional modifications. I'm going to close this PR as it should no longer be needed, but please do take a look at the PR for #16290 once it's available for review and let me know what you think. Thanks again!