patch 9.0.0099: scrollback can be wrong after redrawing the command line

Problem:    Scrollback can be wrong after redrawing the command line.
Solution:   Clear unfinished scrollback when redrawing. (closes #10807)
diff --git a/src/proto/message.pro b/src/proto/message.pro
index d3ec949..3f8a8fe 100644
--- a/src/proto/message.pro
+++ b/src/proto/message.pro
@@ -48,6 +48,7 @@
 int message_filtered(char_u *msg);
 void may_clear_sb_text(void);
 void sb_text_start_cmdline(void);
+void sb_text_restart_cmdline(void);
 void sb_text_end_cmdline(void);
 void clear_sb_text(int all);
 void show_sb_text(void);