Removing and adding back the same file within a DFS transaction causes an error

If within a DFS transaction, a subfile is removed from a superfile and then later added back again, an error is hit:

> addSubFile: File <subfile> is already a subfile of <superfile>

The case it was seen in was:

StartSuperFileTransaction()
ClearSuperFile('super'); // where 'sub' was a subfile of super
AddSubFile('super', 'sub');
FinishSuperFileTransaction();

This is a regression from 3.8.6 onwards