[angular] digest once when adding the k-rebind watcher

Seems that on the first call, newVal === oldVal, even though there was
actually a change.  by digesting we force this useless event to be triggered
on initialization so that it works properly thereafter.  if that makes any
sense.

https://docs.angularjs.org/api/ng/type/$rootScope.Scope mentions:

    “The listener is called only when the value from the current
    watchExpression and the previous call to watchExpression are not
    equal (with the exception of the initial run, see below)” [...]

    “After a watcher is registered with the scope, the listener fn is called
    asynchronously (via $evalAsync) to initialize the watcher. In rare
    cases, this is undesirable because the listener is called when the
    result of watchExpression didn't change. To detect this scenario within
    the listener fn, you can compare the newVal and oldVal. If these two
    values are identical (===) then the listener was called due to
    initialization.”