Issue 4028: Automatic beaming of grace notes inhibits beam subdivision

Issue 4028: Automatic beaming of grace notes inhibits beam subdivision

While the whole extend_mom_ logic seems rather fishy to me (it probably
should be reset to a sane value at some other location per-beam) and
I don't have an actual idea why the Grace_auto_beam_engraver would
be involved at all here, initializing extend_mom_ to a value before
all grace notes seems to do the trick for this report.