Prevent prompts during inccommand previews

For example, "Backwards range given, OK to swap (y/n)?" on each keypress.

src/nvim/ex_getln.c

@@ -1927,7 +1927,9 @@ static int command_line_changed(CommandLineState *s)
     //       - Immediately undo the effects.
     State |= CMDPREVIEW;
     emsg_silent++;  // Block error reporting as the command may be incomplete
+    msg_silent++;   // Block messages, namely ones that prompt
     do_cmdline(ccline.cmdbuff, NULL, NULL, DOCMD_KEEPLINE|DOCMD_NOWAIT);
+    msg_silent--;   // Unblock messages
     emsg_silent--;  // Unblock error reporting
 
     // Restore the window "view".