Make the difflib code ES5 strict mode-compliant

According to http://es5.github.io/#C, "When a simple assignment occurs within strict mode code, its LeftHandSide must not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown (8.7.2)". Also, `var` in the global scope defines properties on the global object (http://es5.github.io/#x10.4.1.1).