Issue 3693: Let Percent_repeat_iterator be unfazed by Timing changes

Issue 3693: Let Percent_repeat_iterator be unfazed by Timing changes

There is still one shortcoming: the percent repeats will not contain any
material apart from the percent itself.  In particular, no Timing
changes will be repeated.  If there are meter changes or \partial
commands inside of percent repeats, they need to occur in parallel
passages outside of any percent repeat, if necessary in a separate
"timing track".