* lily/auto-change-iterator.cc: move contents from

* lily/auto-change-iterator.cc: move contents from
new-auto-change-iterator.cc from

* scm/part-combiner.scm: determine split-list from Scheme.

* lily/parser.yy (part_combined_music): remove old PC cruft.

* ly/engraver-init.ly: remove old
PC cruft.

* Documentation/user/refman.itely (The Lyrics context): note about
extender lines.

* scm/part-combiner.scm (determine-split-list): many bugfixes.

* input/regression/new-part-combine-solo-global.ly: new file.

* scm/part-combiner.scm: rewrite.