upload-archive: handle "-h" option early

upload-archive: handle "-h" option early

Normally upload-archive forks off upload-archive--writer to
do the real work, and relays any errors back over the
sideband channel. This is a good thing when the command is
properly invoked remotely via ssh or git-daemon. But it's
confusing to curious users who try "git upload-archive -h".

Let's catch this invocation early and give a real usage
message, rather than spewing "-h does not appear to be a git
repository" amidst packet-lines. The chance of a false
positive due to a real client asking for the repo "-h" is
quite small.

Likewise, we'll catch "-h" in upload-archive--writer. People
shouldn't be invoking it manually, but it doesn't hurt to
give a sane message if they do.

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