[fontdiff] Designate choices for diff-type arg
[fontdiff] Designate choices for diff-type arg

anthrotype/nototools,anthrotype/nototools,googlefonts/nototools,moyogo/nototools,googlefonts/nototools,dougfelt/nototools,googlei18n/nototools,moyogo/nototools,googlefonts/nototools,googlefonts/nototools,anthrotype/nototools,googlei18n/nototools,dougfelt/nototools,googlei18n/nototools,moyogo/nototools,dougfelt/nototools,googlefonts/nototools