Grader: do not allow unlimited (-1) line range
Grader: do not allow unlimited (-1) line range

As passing -1 to either parameter would render the flag ineffective, it
is not necessary to allow this edge case if one could just omit it.

cksystemsteaching/selfie,cksystemsteaching/selfie,ChristianMoesl/selfie,ChristianMoesl/selfie,cksystemsteaching/selfie