debugrebuildstate: clarify that rev can't be specified without -r -r has a default value of '' in the command line. The function default value of 'tip' is thus never used and any attempt at specifying revisions without -r will fail. It seems like then intended behavior was that 'hg debugrebuildstate' without any parameters should set the parents to tip. That would be very confusing now when the command primarily is used to recover from incorrect stat info. It is apparently undocumented that '' is the same as '.' ... unless it is passed in a place where revsets are used.