Music_iterator::process timekeeping

[Dan Eble]

I have some questions about this.

> void
> Auto_change_iterator::process (Moment m)
> {
>   Music_wrapper_iterator::process (m);
> 
>   Moment now = get_outlet ()->now_mom ();
>   Moment *splitm = 0;
>   if (start_moment_.main_part_.is_infinity () && start_moment_ < 0)
>     start_moment_ = now;

1. Is there any input for which m != now?  Favoring an implicit parameter (now) 
over an explicit parameter (m) smells fishy, but that’s about all I can say at 
the moment; the iteration code is rather complicated, and there are not many 
comments to help me.

2. Is there any input for which start_moment_ != this->music_start_mom()?

I tried using “m” and “music_start_mom()” in lieu of the above, both in 
Auto_change_iterator and in Part_combine_iterator.  I found no difference in 
any of the regression tests.  You can see my experimental changes at 
https://codereview.appspot.com/242900043/ .

Thanks,
— 
Dan