clone: validate --origin option before use

clone: validate --origin option before use

Providing a bad origin name to `git clone` currently reports an
'invalid refspec' error instead of a more explicit message explaining
that the `--origin` option was malformed.  This behavior dates back to
since 8434c2f1 (Build in clone, 2008-04-27).  Reintroduce
validation for the provided `--origin` option, but notably _don't_
include a multi-level check (e.g. "foo/bar") that was present in the
original `git-clone.sh`.  `git remote` allows multi-level remote names
since at least 46220ca100 (remote.c: Fix overtight refspec validation,
2008-03-20), so that appears to be the desired behavior.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Derrick Stolee <stolee@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Sean Barag <sean@barag.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>