What part of "do this only if blobWalk is null at the start of a loop" did you not understand from my comments on patch set 18?

Files are renamed infrequently. Always looking for rename information on every commit is horrifically slow. Do it only when the file suddenly disappears on you. When the file is gone, either its because its history is over,  or its because the file was renamed. Its only worth doing the rename detection when its gone.