VcsCommand: Block auto reload while a command is running

Git rebase is executed in the background, and it can change a file multiple
times in a short period. If we reparse a file while this happens on
Windows, Git fails to replace it, the rebase action fails and the
repository becomes unstable (remains with a modified file).

See discussion at https://github.com/git-for-windows/git/pull/1666

Task-number: QTCREATORBUG-15449
Change-Id: Iba40a770a1df2dfff0dd1c874c491dfbe1cceb58
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/shellcommand.h

@@ -154,6 +154,7 @@ public:
 signals:
     void stdOutText(const QString &);
     void stdErrText(const QString &);
+    void started();
     void finished(bool ok, int exitCode, const QVariant &cookie);
     void success(const QVariant &cookie);