Remove empty human Subjects when removing changes made by another human

[janehmueller]

When a human Subject is edited in a Commit, it is compared with the master Subject. This master Subject already contains the changes of the human Subject. Therefore a second change that reverts a previous change by the human Subject would also be a new change that is written to the human Subject.

Every change should be compared with the other slave Subjects to see if the human Subject, or the value in a specific fields, can be simply removed.