remote-{ext,fd}: print usage message on invalid arguments

remote-{ext,fd}: print usage message on invalid arguments

We just say "Expected two arguments" when we get a different
number of arguments, but we can be slightly friendlier.
People shouldn't generally be running remote helpers
themselves, but curious users might say "git remote-ext -h".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>