Fix ChangeSingleBarLineSpan class to cope with start repeats

When the span of a start repeat bar line at the beginning of a measure is changed, the bar lines of the following staves are not adjusted.

Note: this PR may be not automatically merge-able with https://github.com/musescore/MuseScore/pull/594 as both modify undo.h/.cpp in very near spots; if the decision to merge them is made, I'll manage the potential conflicts.