Added 'include_repeated_pitches' property to Glissando.

OLD: Glissando always included repeated pitches.
NEW: Glissando includes repeated pitches when include_repeated_pitches=True.
Glissando excludes repeated pitches when include_repeated_pitches=False.

Glissando now defaults to include_repeated_pitches=False.

Set include_repeated_pitches=True to recoup the old behavior.

Doesn't need to be included in the change log.