Draw message as one format, allowing prompts and messages to occupy only

a portion of the status bar, overlaying the normal status content rather
than replacing the entire line. A new message-format option now controls
the entire message (like status-format). From Conor Taylor in GitHub
issue 4861.

tmux.h

@@ -911,6 +911,7 @@ struct style {
 	char			range_string[16];
 
 	int			width;
+	int			width_percentage;
 	int			pad;
 
 	enum style_default_type	default_type;