fixes casting bug in diffLineSwapOk
fixes casting bug in diffLineSwapOk

RCOS-Grading-Server/HWserver,Submitty/Submitty,Submitty/Submitty,RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,Submitty/Submitty,RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,Submitty/Submitty,RCOS-Grading-Server/HWserver,RCOS-Grading-Server/HWserver,Submitty/Submitty,Submitty/Submitty,Submitty/Submitty,Submitty/Submitty,RCOS-Grading-Server/HWserver,Submitty/Submitty