kdb: Tweak escape handling for vi users

Currently if sequences such as "\ehelp\r" are delivered to the console then
the h gets eaten by the escape handling code. Since pressing escape
becomes something of a nervous twitch for vi users (and that escape doesn't
have much effect at a shell prompt) it is more helpful to emit the 'h' than
the '\e'.

We don't simply choose to emit the final character for all escape sequences
since that will do odd things for unsupported escape sequences (in
other words we retain the existing behaviour once we see '\e[').
Signed-off-by: NDaniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: NDouglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191025073328.643-6-daniel.thompson@linaro.org

kernel/debug/kdb/kdb_io.c

@@ -163,8 +163,8 @@ char kdb_getchar(void)
 
 		*pbuf++ = key;
 		key = kdb_handle_escape(buf, pbuf - buf);
-		if (key < 0) /* no escape sequence; return first character */
-			return buf[0];
+		if (key < 0) /* no escape sequence; return best character */
+			return buf[pbuf - buf == 2 ? 1 : 0];
 		if (key > 0)
 			return key;
 	}