credential: handle invalid arguments earlier

credential: handle invalid arguments earlier

The git-credential command only takes one argument: the
operation to perform. If we don't have one, we complain
immediately. But if we have one that we don't recognize, we
don't notice until after we've read the credential from
stdin. This is likely to confuse a user invoking "git
credential -h", as the program will hang waiting for their
input before showing anything.

Let's detect this case early. Likewise, we never noticed
when there are extra arguments beyond the one we're
expecting. Let's catch this with the same conditional.

Note that we don't need to handle "--help" similarly,
because the git wrapper does this before even calling
cmd_credential().

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