feedback: show a message while executing an external command

Some commands can take a little while to execute; showing just the prompt
during that time could give the impression that nothing is happening.

This addresses https://savannah.gnu.org/bugs/?56041.

src/text.c

@@ -1019,6 +1019,8 @@ bool execute_command(const char *command)
 		return FALSE;
 	}
 
+	statusbar(_("Executing..."));
+
 	/* If the command starts with "|", pipe buffer or region to the command. */
 	if (should_pipe) {
 		linestruct *was_cutbuffer = cutbuffer;