an acceptance condition on all outgoing transitions.

This was motivated by experiments from Rüdiger Ehlers, showing
that "ltl2ba -f 'a U (b U c)'" outperformed "ltl2tgba -f -N -R3 'a
U (b U c)'".  With this change and the previous one, it is no
longer the case.

* src/tgba/tgbatba.cc (tgba_tba_proxy_succ_iterator::aut_): Store
a pointer to the source automaton and...
(tgba_tba_proxy_succ_iterator::sync_): ... use it in an extra
optimization step to gather the acceptance conditions common
to all outgoing transitions of the destination state, and pretend
they are on the current (ingoing) transition.
(tgba_tba_proxy::succ_iter): Pass the
source automaton to the constructed iterator.
* src/tgbatest/spotlbtt.test: Test -f -N -R3 -r7.
* src/tgbatest/ltl2tgba.test: Add a test case for 'a U (b U c)'.