#63 Auto bar-check display confused by "\change" indications

[Valentin Villenave]

When typesetting a music fragment that includes such things as
\change Staff = "foobar"
the bar-check count that is automatically displayed when typing the | character gets confused;

\new Staff = "thisStaff"
\relative c {
c d
\change Staff = "thisStaff"
e f | % prints "1/2"
\change Staff = "thisStaff"
g1 | % prints "5/4"
}