Do it when we reach 50% of the difference between fPruneLength and fMaxDepth: this means the history file can contain more lines than fMaxDepth (if multiple processes write to it sort of synchronously), but not many (getting them to work synchronously over hundreds of lines is rare).


git-svn-id: http://root.cern.ch/svn/root/trunk@39533 27541ba8-7e3a-0410-8455-c3a389f83636