fetch-pack: check for shallow if depth given

When a repository is first fetched as a shallow clone, either by git-clone or by fetching into an empty repo, the server's capabilities are not currently consulted. The client will send shallow requests even if the server does not understand them, and the resulting error may be unhelpful to the user. This change pre-emptively checks so we can exit with a helpful error if necessary. Signed-off-by: Mike Edgar <adgar@google.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>