Revision 39539

Import of r39533 from trunk.

Don't open the history file for enery entered line, to determine its number of lines.
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).