The --origin option comment makes no sense in this context.  We don't use this string within the project reference in the project set, but I expected you to be using it given your comment right here.

Also, we're probably OK to just hardcode this as "origin" via the DEFAULT_REMOTE_NAME constant.  Users aren't likely to change it.  IMHO project set files are about simplifying getting started with a project in Eclipse.  We should help that by also setting some basic standard rules, like the name of the default remote that the project set created.