This was slightly less confusing, because it at least
mentions that there are two ways to invoke (but it still
isn't clear why what the user provided doesn't work).
Instead, let's show an error message like:
$ git mv one two three
fatal: destination 'three' is not a directory
We could leave the usage message in place, too, but it
doesn't actually help here. It contains no hints that there
are two forms, nor that multi-file form requires that the
endpoint be a directory. So it just becomes useless noise
that distracts from the real error.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>